NEW YORK (AP) _ You know a show is in trouble when the audience doesn't realize it's over until the actors suddenly start taking their bows.

It's the most startling moment in ''Have I Got a Girl for You 3/8,'' a silly, inept musical send-up of the Frankenstein legend that opened Wednesday at off- Broadway's Second Avenue Theater.

Early on there are plenty of clues that it will be a long evening of theater. Beware of any musical that opens with German peasants singing joyfully about death. In this case, they are celebrating the demise of the monster created by Baron von Frankenstein.

It's a false alarm, and the creature arrives to scare the stuffing out of the local farm folk. Don't expect Boris Karloff. This monster is sort of a beefcake behemoth who wears lederhosen and has metal bolts attached on either side of a neck the size of New Jersey.

What this Bavarian beach boy is doing here never is quite clear. The center of the musical is Frankenstein's dimwitted, buck-teethed nurse, a good-hearted medical technician named Mary Phillips. Frankenstein has designs on her money and will even go so far as to marry her to get it.

A more important plot line goes in a decidedly different direction. Authors Joel Greenhouse and Penny Rockwell have added a second evil physician, Dr. Pretorius, who wants to create the ultimate Hollywood star, using body parts from famous movie actresses - for example, Kitty Carlisle's heart (her married name is Hart). That's a sample of the kind of humor that peppers the dismal story.

Dick Gallagher's occasionally witty score includes several bouncy tunes, especially a title song that skillfully suggests several 1930s pop hits. The score and the story collapse in the second act, when Nurse Mary ends up on the operating table and is turned into a sequined version of Elsa Lanchester in ''The Bride of Frankenstein.''

Director Bruce Hopkins abandons his actors on stage, stranding them at the worst moments, usually right after a joke that has fizzled out. There are long, awkward patches between scenes that have actors parading on and off stage like participants in a high school pageant.

The cast either overacts or can't act at all. The one exception is Semina de Laurentis as the intrepid nurse. She cheerfully sails through the musical with the confidence of a true believer in the material. Miss de Laurentis has a peculiar tremelo that she uses to good effect, particularly in ''Always for Science,'' her lament for the uncaring doctor.

The bargain basement sets by Harry Darrow are shabby and unconvincing. They might look better in a smaller house but the Second Avenue Theater is much too large for this small-scaled, small-minded show. ''Have I Got a Girl for You 3/8'' is an offer everyone should refuse.