CBS Inc. has agreed in principle to underwrite a substantial part of the cost of the Kennedy Center's 36-week season of six plays, Center chairman Roger L. Stevens said yesterday.

Although no contracts have been signed, the two organizations have agreed to what CBS senior vice president Tom Leahy called a "co-venture." Acording to Stevens, CBS will finance the six plays with the Kennedy Center, at a joint cost of about $1 million. The plays are being produced under a LORT (League of Resident Theaters) contract and the profit potential for CBS is limited. Reports that CBS would get film or television rights to the productions are not accurate, Stevens said, because under the LORT contract such rights are not owned by the producers.

According to Thomas R. Kendrick, the Kennedy Center director of operations, "If CBS were interested in obtaining the rights necessary to take one of these plays to Broadway, and possible future television and film development, the Kennedy Center would use its best efforts to help them obtain the rights, and might choose to participate in such a production."

"CBS came to us and wanted to finance some shows," Stevens said from New York, where he is recuperating from open-heart surgery. "I guess they hope some material would come out of it."

"We are an industry based on creativity," said Leahy. "And anything we can do to expose or enhance the development of creativity is to our mutual advantage."

By operating under an LORT contract, the Kennedy Center can produce plays for less than the cost of producing them under a "first class" contract, Stevens said. Salaries and royalties are less under the LORT, he said, but so are the producers' rights to future Broadway, film or television productions of the plays. He said the "first class" contract also required the Kennedy Center to take some show to Broadway "whether we wanted to or not." About 40 shows that opened in the Eisenhower Theater have gone on to Broadway in the last 10 years, not all of them produced by the Kennedy Center, and not all of them money-makers.

The season's performers will include actors who will "perhaps" appear in smaller roles in more than one production as well as established stars who will only appear in one show, Stevens said. He said the season of plays will not be given a name or identified as being performed by a company. "It's the same thing we've done in the 10 years we've been producing plays at the Kennedy Center," he said. "The only thing that's different is that it's a LORT contract . . . . I hope that is due course we can give it a separate identity."

The season, which is expected to open in December, has not been set, but company director Ralph Allen has said he hopes to get stars such as Robert Redford, Paul Newman, Jason Robards and Jack Lemmon. Variety reported this week that a production of "Idiot's Delight" starring Burt Reynolds was also being discussed. Stevens said all of the plays under consideration were revivals and the selection of each play would be tied to the interest of a particular star.

"These productions can pay a star no more than about $2,500 a week, and that's nothing to them, so they'd only do something they'd like to do," Stevens said, noting that the Music Center in Los Angeles has had considerable success with a similar arrangement.

Stevens has long hoped to start a national resident theater in Washington comparable to Britain's National Theater, which attracts the cream of British performers. A previous project with New York producer Joseph Papp fell through, but Stevens said that even if the deal with CBS collapses, the season of six plays will go on as planned.