Clinical studies were carried out with laboratory female rats to ascertain the effects of daily dosing with MPA (medroxyprogesterone acetate). Some of the rats received .8 mg MPA in 1.4 ml acqueous suspension. The others received 2 mg MPA in intramuscular injections. The doses used were those necessary for estrus synchronisation. Body weight was increased by MPA given by gastric intubation. Liver weight, dry matter, protein, DNA, and RNA were not significantly affected by daily MPA, either by intubation or by injection. Adrenocortical atrophy occurred as a result of the MPA administration. This response was dose-dependent. Histologically, the loss of adrenal weight was due almost entirely to the involution of cortical cells. The mechanism of adrenocortical atrophy involved extensive single cell deletion, due perhaps to the suppression of pituitary ACTH secretion. No changes in the liver were observed through biochemical determinations.