An agreement by the television networks to present viewer advisories warning of programs that contain violence will be unveiled here today at a Capitol Hill press conference,according to an industry source.

Executives of ABC, CBS, NBC and Fox will be on hand to announce a landmark “experiment” to place parental advisories where appropriate in advance of programs that might be offensive to viewers.

The agreement will last two years and does not include a commitment to reduce levels of vid violence, the source said.

Also participating will be Jack Valenti, prez of the Motion Picture Assn. of America, as well as Sen. Paul Simon (D-Ill.) and Rep. Edward Markey (D-Mass.). The two lawmakers are expected to hail the experiment as a long-awaited measure and to urge others in the broadcast and cable TV industry to follow suit.

Exex on hand for the press briefing will be Thomas Murphy, chairman of Capital Cities/ABC; Howard Stringer, prez of CBS Broadcast Group; Warren Littlefield, prez of NBC Entertainment; and George Vradenburg, exec veepee of Fox Television.

The announcement comes roughly one month before an industry conference in Los Angeles on TV violence. Slated to include participation of MPAA-member companies , it will be the first use by the television industry of a 1990 law that exempts it from antitrust laws so it can develop standards on TV violence. The law, authored by Simon, expires in December.

The move was applauded by James Quello, chairman of the Federal Communications Commission. “Any kind of self-regulation is a step in the right direction,” he said. “If the networks don’t do it, they are vulnerable to legislation similar to one recently passed on indecency.”