IT was not too long ago when consumers could easily appease their creditors by simply advising, ''The check is in the mail.'' But the Internet is making it harder to come up with excuses.

About a dozen person-to-person online payment services -- from systems offered by Citibank and Western Union to a Web-based venture called PayPal -- have emerged over the last year or so. They allow consumers to transfer money quickly online through their bank accounts or credit cards, whether to settle a dinner check, send spending money to a college student or pay for an online auction purchase. Recipients are notified by e-mail, sometimes with the proclamation, ''You've got cash.''

''It's one of the fastest-growing Internet services out there,'' said Paul Jamieson, director of financial services at Gomez Advisors, a research firm based in Lincoln, Mass., that ranks e-commerce sites.

The idea of person-to-person payment services, which have both good and bad points, found its footing in online auctions, where a speedy payment system is crucial to completing fast-paced transactions.

Jenanne Hassler, who is both a buyer and a seller on the eBay auction site and earns her living selling lingerie over the Internet, said she started using PayPal for her online bidding purchases. Ms. Hassler, 32, of Waynesville, N.C., now uses it routinely for a variety of payments -- even to send cash birthday gifts to friends. ''To me it seems like the perfect and most convenient way to say, 'I love you; here's $15,' '' she said. ''It's very quick and very easy.''

Gary J. Caruso, commissioner of the Congressional Softball League since the early 1980's, said that this year, the league is offering PayPal as an option for paying fees. ''We have a postal box and sometimes the mail just disappears,'' he said, adding that one 1999 registration and fee did not show up in the regular mail until the fall of 2000.

Most of the online services use the same basic principle. Both parties to a transaction must have e-mail addresses. The person sending the money registers with the service to establish an account and provides the number of a credit card or bank account from which the money will be drawn. After the sender pays, the recipient receives an e-mail notification. In most cases, the recipient must also register with the service and then designate a method for receiving payment. The money can be transferred into a bank account, credited to a credit-card account or mailed by check.

Generally, recipients can also hold the money in an online account set up with the service at registration. The money can then be transferred around the Internet and used for other purposes.

Although a transaction is noted immediately in a recipient's e-mail, it usually takes about three days for the money to actually arrive in a bank account and from 7 to 10 days for a check to arrive through the mail.

To address that delay, some companies have started issuing debit cards, which allow recipients to gain access to their funds immediately through automated teller machines.

Ms. Hassler, who recently received her debit card, uses her PayPal account to accumulate payments from her eBay sales. She says she eventually will use the account for everyday purchases like groceries.

While the offerings are generally similar, pricing is different.

PayPal, which was started 15 months ago and is a leader in the field with 7.5 million users, actually pays users $5 to register; the basic service is free for personal use. Businesses or individuals who sign up for a ''premier account,'' which includes additional features like debit cards, pay 30 cents for payments of $15 or less and 30 cents plus 2.2 percent of the amount on payments over $15.

Vince Sollitto, a spokesman for PayPal, said that nearly 80 percent of the accounts belonged to businesses and that 50 percent of their traffic came from online auctions.

The c2it service from Citibank, a unit of Citigroup, started a year ago. It is free for the first 90 days and then costs $2 a transaction. But the bank says it plans to change its pricing structure eventually to a small flat fee plus a percentage of the transaction.

MoneyZap, Western Union's online person-to-person payment system, began last July and is free for now. But the company, a unit of First Data, says it is developing a payment structure.

Both c2it and MoneyZap have 30,000 to 40,000 users, according to analysts. Western Union, which has been in the money transfer business for 150 years and had 89 million transactions last year, says volume at MoneyZap is growing about 10 percent a week.

Whatever the fees, industry analysts say the cost of Internet payment services is a fraction of that of old-fashioned bank money transfers, which run from $10 to $25.

''It's really leveraging the power of what the Internet does best -- communicating and sending information,'' Mr. Jamieson said. ''In this case, money is the information.''

While online payment systems are generally viewed as secure -- Mr. Caruso says that even the captain of the General Accounting Office softball team has found it safe and easy to use -- analysts warn that there are some risks. They say that a user, when paying for such a transaction with a credit card, is typically not covered by fraud protections that commonly come with credit-card purchases. In general, they say, those rights are waived because the transaction is viewed as sending cash to someone.

''It's really like transferring cash around the Internet,'' Mr. Jamieson said. ''Know that individual you are doing business with. Know you may not have any recourse if you transfer funds to them.''

James Van Dyke, a senior analyst at Jupiter Media Metrix, an Internet research firm, said the old system of sending money was not foolproof, either. ''In the old world, you were required to send a check,'' he said. ''Somebody had to trust somebody.''

Avivah Litan, vice president for payment services at the Gartner Group, a technology consulting firm, said she saw a bright future for the online payment services, especially if they were combined with wireless technology, allowing consumers to make purchases and payments with their cell phones, for example.

''My prediction is that these companies that are going to offer these new applications won't want to use traditional banking payments,'' she said. ''It's a model that isn't going to go away.''

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