The authors treated three groups of rats with medroxyprogesterone acetate (MAP) at different dosages (0, 2.5, 5, 10, 20, 40, 80 mg/kg) i.p. and orally for 7 days to evaluate its anabolic and its androgenic/antiandrogenic effect. 70 rats had been previously castrated and 35 were intact. The anabolic effect is evident in all groups of animals which were treated. The androgenic action predominates in castrated animals, while the antiandrogenic action predominates in intact animals. The above-mentioned actions are more marked when very high doses are used. After suggesting the possible mechanism of action of MAP, the authors point out that the anabolic effect confirms what has already been reported in clinical oncology after treatment with very high doses of MAP (1,000-2,000 mg orally or i.m. for 30 days) in different types of tumors (breast, kidney, prostate).