Glycine is used to treat various health problems and is efficient in the treatment of the negative symptoms of schizophrenia. Since glycine exists as a few polymorphs, the aim of this work is to compare the effects of the alpha- and gamma-forms of glycine on the behavior of the genetic catalepsy (GC) strain of rats. Both polymorphs of glycine have been administered to rats orally as pure solid chemicals, and cataleptic behavior and behaviors in the open-field, elevated plus-maze, and light-dark box tests were studied. Both the alpha- and gamma-polymorphs of glycine increased exploratory activity in the open-field test, but only the gamma-polymorph had beneficial effects on catalepsy and exploratory activity in the light-dark box and reduced anxiety in the elevated plus-maze.