For those who might be wondering what Betty, Bert and the Dope King of Providence, R.I., are smoking in "Landscape of the Body," the folks at the Studio Theatre insist it isn't really marijuana, but rather a concoction of various herbs -- catnip, wild lettuce, periwinkle and peppermint -- that, when burning, smells suspiciously like the controlled substance.

The "pot" is the invention of actor Rick Venable, who is also a chemist, and debuted in 1983 at the Studio in "Fifth of July." Michael Wells, who plays Bert in "Landscape," admits to inhaling the stuff in the name of artistic realism, but says, "It hasn't affected me in any way."

And speaking of illusions in "Landscape," pedestrians in the vicinity of the 1400 block of Church Street NW should be warned that the decapitated body in the shopping cart that comes hurtling out of the Studio's emergency exit Wednesday through Sunday nights around 10 is not real either. Washington Theatre Awards

Kathy L. Dwyer was recently named president of the Washington Theatre Awards Society -- presenters of last year's Helen Hayes Awards -- which also added eight new members to its board of directors, bringing its total to 24. Dwyer, who currently serves as the program officer of the Eugene and Agnes E. Meyer Foundation, takes over from founding board member and past president Richard Bryant, who this week became managing director of the Arizona Theatre Company.

Nominees for this year's awards will be announced in mid-March, with the presentation to be held later this spring. 'Shady Side' Screen Rights

The screen rights to Stephen Hayes' play "The Shady Side," which was produced at the Source Theatre in the fall of 1984, have been acquired by John Simmons, a local television producer, and local financier Charles Sutherland. Simmons will coproduce with Fred Caruso, whose previous films include "Blowout" and "The Year of the Dragon."

Hayes will write the film script for his play, which focuses on a former stage queen, her pop-psychologist husband and a psychopathic bodyguard who spend a weekend in a Maine lighthouse.

Simmons said he has already received several offers of financing and that filming will probably begin this year at a yet-to-be-named East Coast site. 'Ma Rainey' at the Studio

Samuel French giveth, and Samuel French taketh away . . . And Samuel French giveth again? That appears to be the case as the New York literary agency, after retracting the rights to "The Normal Heart" (at the request of playwright Larry Kramer) from the Studio Theatre, has now given the Studio the opportunity to mount the first production of "Ma Rainey's Black Bottom" since it ended its successful run on Broadway last year. The play, which centers on racism in the music world of the 1930s, opens May 14 and will be the Studio's final show of the season. Odds and Ends

Wayne Knickel, who for the last five years has been the associate director of public relations for the National Theatre, yesterday became director of public relations for Baltimore's Morris Mechanic Theatre, with additional responsibilities over the Lyric Opera House and the Pier Six Concert Pavilion. Knickel assumes the post from Norman Zaiger, who becomes press representative for the international tour of "Dreamgirls," which arrives at the National for a seven-week engagement April 22 . . . The Studio Theatre and its Acting Conservatory received a two-year, $20,000 grant from the Philip L. Graham Fund and a one-year, $10,000 grant from the D.C. Commission on the Arts, which will be used to increase compensation to artists, produce new plays and expand the scholarship program and classroom facilities . . .Maryland Governor Harry Hughes has declared this week "Maryland Community Theater Week," honoring this weekend's tournament of one-act plays by amateur groups from around the state, which will be held at the Performing Arts Center on Montgomery College's Rockville campus; call 279-5301 . . . The Stratford Festival of Canada performs "Twelfth Night" and "King Lear" in repertory for one week at the Warner Theatre beginning tonight.

Extended Runs: "The Light" through Feb. 8 at the Source's Warehouse Rep; "Secret Honor" through Feb. 16 at New Playwrights'.

Opening this week: Tonight, "Zorba," starring Anthony Quinn and Lila Kedrova, at the National Theatre; Friday, "The Gin Game," starring husband-and-wife team Arthur Peterson and Norma Ransom, at George Mason University's Harris Theatre.