Moss Hart, veteran playwright and director, has apparently been collecting gripes for several years. The theatre is richer for this warehouse. For at the Plymouth Theatre he releases these repressed emotions like so many small birds in an entertaining and often hilarious new comedy, "Light Up The Sky."

This latest Hart gem centers about two antagonistic groups: a playwright on the one hand, and a crew of theatrical angels, directors, and actresses on the other. And it would appear that Mr. Hart has felt for too long that 1) playwrights fear to say what they believe with daring and originality, and that 2) theatrical folk are tough and reasonably unscrupulous cutthroats. All this Mr. Hart has now said.

With Moss Hart, it isn't the plot that counts, but the trimmings. Actually the story is concerned with a bright young man who has switched from trucks to typewriters and has written something new in the way of drama. This young man finds that the people who once seemed to be his devotees desert him when his play appears headed for failure. The playwright then loses his faith in his play as well as in his associates; but eventually he becomes as tough as the rest of them, and goes on with the job. The plot is slow in starting, and in some of the later serious moments it appears to get in the way of the real guts of the show, which are delightful.

And the guts, as said before, are the trimmings. These include two characters obviously patterned after Billy Rose and his wife Eleanor Holm. Sam Levene and Audrey Christie do a fine job of making these two into tough, witty, shrewd people, the kind Hart loves to harrass. Virginia Fields, who looks better than ever, portrays a shifty Lady in Lights who gurgles "darling" to almost everyone but her dull-witted Wall Street husband, obviously another pet peeve of Hart's. For only two major characters does the Hart show tenderness. One is the playwright in the plot, played earnestly and well by Barry Nelson. The other is the actress' mother (Phyliss Povah), slandered often, but always ready to play Gin with the play's angel.

"Light Up The Sky" can not compare with "The Man Who Came To Dinner," on which play Hart collaborated with the great comic genius, George S. Kaufman. But the new play is like the earlier masterpiece in that both shows hit their strides when they insult people. In "Light Up The Sky," directed by Hart himself, the insults come crisp and clean and funny. If Hart can now grease up the serious portions of the show, Broadway's big brass will be in for vigorous punishment for several months to come.