AOL Time Warner Inc. is giving some users of its AOL and ICQ messaging services the ability to send messages to each other for the first time, as part of an experiment to no longer treat the programs as separate services.

The two instant-messaging services have served different audiences. ICQ says it has 135 million accounts, mostly abroad. AOL Instant Messenger, or AIM, is the dominant service in the United States, with 180 million registered accounts. Many users maintain multiple accounts.

Subscribers have been unable to send messages between the two free services even though the programs are owned by the same company. AOL Time Warner is conducting a limited test to gauge demand for connecting the two and to refine the technology.

"ICQ is available in 18 different languages, and it is wildly popular in Europe. It is one of these viral phenomena," said AOL spokeswoman Anne Bentley. "AOL members want the ability to exchange instant messages with ICQ members," she added.

Bentley said the experiment should not be confused with industry efforts to agree on a general standard for interoperability of separate instant-messaging programs. AOL says it is worried that such connections could compromise the security of its systems and expose users to more unwanted appeals from strangers and other junk messages.

Because AOL and ICQ are handled by the same computer network, America Online is comfortable testing whether its systems can be linked, Bentley said.

The test, details of which circulated on Web sites yesterday, is similar to a new arrangement that allows users of Apple Computer Inc.'s iChat program to send instant messages to AOL members. AOL's network hosts the iChat communications, just as it maintains control over ICQ.

Instant messages differ from e-mail in that the notes are traded almost instantaneously, allowing users to converse in writing almost as they would by telephone. The systems allow users to set up "buddy lists," which indicate when friends are online. That is important because, unlike e-mail, instant messages can be received only when their recipients are online.

As the dominant player in instant messaging, AOL gives no indication that it wants to adopt a more open messaging architecture that would allow contact with users of Microsoft Corp.'s MSN Messenger or other services. A group of Wall Street firms have urged AOL and Microsoft to collaborate on a system for the financial services industry. Bentley said there are significant technological hurdles to adopting a common system.

Some say AOL is moving too slowly to make the service more accessible. "Instant messaging has the possibility to be the e-mail killer, but not until there is one instant-messaging standard that everyone can use," said software developer Eric Peyton.

When America Online and Time Warner merged, AOL had such a commanding position in instant messaging that the Federal Communications Commission restricted its ability to expand AIM to include music, video and other features if it remains a closed system. AOL's rivals said they feared it would use its dominance to turn its instant-message service into a platform for online services.

Since acquiring ICQ Inc. in 1998, AOL has operated it as a separate business and repeatedly thwarted efforts by third-party developers to connect AOL members with ICQ users and others. Bentley said AOL regards those third parties -- including a service called Trillian, built by Cerulean Studios -- as "hackers" that threaten the security of AOL members and the AOL network.

Under AOL's test, America Online members and AIM users are able to list ICQ members as instant-messaging "buddies," but ICQ members cannot do the reverse. It could not be determined how many people are participating in the test. "We have removed the virtual glass wall," Bentley said.