Twelve cancer patients and one patient with diabetes mellitus were treated with medroxyprogesterone acetate (MPA) by intramuscular injection in a total weekly dose of 400, 700, or 1200 mg. The treatment reduced the plasma cortisol concentration by 76% in the AM hours (21 leads to 5.0 mug/dl) and by 75% in the PM hours (12.8 leads to 3.2 mug/dl). Cortisol production rate decreased by 67% (19 leads to 6.2 mg/24 hrs). The 24 hour profile of plasma cortisol concentration measured in 3 patients showed zero secretion over this period. Low plasma ACTH values prevailed during treatment, and a blunted response to maximal ACTH stimulation was found. No evidence of adrenal insufficiency was observed in any patient, even though in some patients the plasma cortisol concentration remained at zero for many weeks. MPA has cortisol-like effects and the suppression of adrenal function is probably mediated by a negative feedback action on the hypothalamus or pituitary.