The long-planned production of Tennessee Williams' "The Night of the Iguana," starring Jeanne Moreau, will be coming to Baltimore's Morris A. Mechanic Theatre in October. Moreau, one of France's most renowned actresses, will play the New England spinster in her Broadway debut. The play costars Michael Moriarty and an as yet unnamed star to play the widow Maxine. It will be directed by Arthur Sherman, who was nominated for a Drama Desk award for the recent revival of "The Caine Mutiny Court Martial."

The Mechanic's season also includes the pre-Broadway tryout of "Back on the Town," which marks Nancy Walker's return to the musical stage after her sojourn in television land. Her costar is Liliane Montevechi, the elegant chanteuse from "Nine." The show, a revue conceived and directed by Martin Charnin, is scheduled to open Jan. 7.

The rest of the season includes a new comedy thriller "Corpse!" by Gerald Moon, starring Keith Baxter and Milo O'Shea; "Dreamgirls," which will also play here at the National; "42nd Street," and "Biloxi Blues," Neil Simon's companion piece to "Brighton Beach Memoirs." Also "La Cage aux Folles," which will have been here also, and "Noises Off." . . .

More season news: The New Arts Theatre, which devotes itself to contemporary theater, will open in October with "and a Nightingale Sang," by C.P. Taylor, which deals with a British family in the aftermath of World War II. That will be followed by "Whale Music," by Anthony Minghella, about a group of women awaiting the birth of one friend's child. "Savages," Christopher Hampton's play about an English diplomat kidnaped by guerrillas while a tribe of Brazilian Indians is being slaughtered, follows. The season concludes with "Dorothy," by Graham Reid, set amid the violence in Belfast. Call 654-8332 for more information . . .

Horizons will open its ninth season with "Miss Lulu Bett," which won the Pulitzer Prize for drama in 1920. It was written by Zona Gale and concerns a middle-class spinster. The playwright provided two endings to the play, which will be presented on alternate nights.

The group's second production will be either "Automatic Pilot" by Erika Ritter, which is about a stand-up comedienne, or "It Had to Be You," by Renee Taylor and Joseph Bologna, about a would-be actress and the producer she holds hostage in her apartment.

Their third production will be "American Beef," by Elizabeth Diggs, who wrote last season's "Close Ties." It is set in the Southwest and involves three generations on a family ranch. Call 342-7706 . . .

Ford's Theatre will open in September with "Handy Dandy," a new comedy by William ("Two for the Seesaw") Gibson, starring James Whitmore and Audra Lindley. It's about the relationship between a conservative judge and a social activist nun. Then Barbara Cook will try out her new musical revue at Ford's in October. It's called "Barbara Cook: A Broadway Evening," and is directed by Cook's longtime associate, Wally Harper.

A revival of "Little Me," directed by new artistic director David H. Bell, will open in November for an undetermined run, and the rest of the season will be announced later. Odds and Ends

Marie Baker, formerly executive director of the Montgomery County arts council, has been named director of development for the National Institute for Music Theater . . . Arena's Douglas Wager will direct a production of "Execution of Justice" in New York . . .

Courtesy of Variety, a few other items: Glenda Jackson will repeat her role in "Strange Interlude" for a $6 million, six-hour mini-series scheduled to start filming next spring . . . Britain's National Theater has canceled a revival of "The Entertainer" after playwright John Osborne refused to let Joan Plowright play one of the leads. Alan Bates was to play Archie Rice, Plowright's husband's favorite role. It may be scheduled for 1986 . . .