Medroxyprogesterone acetate: peripheral serum levels and its effects on pituitary and ovarian functions in the rhesus monkey.

17 alpha-Acetoxy-6 alpha-methylprogesterone (medroxy-progesterone acetate, MPA) in doses of 50 mg followed 4 weeks later by an additional 100 mg was i.m. injected in 5 adult normal female monkeys. Serum levels of MPA, progesterone, estradiol-17-beta and LH were determined every other day for a period of 8 weeks then weekly for 8 months. Peak values of serum MPA were detected 2 weeks postinjection followed by a gradual decline to undetectable levels after variable periods ranging from 125 to 225 days. The drug, in the doses used, caused inhibition of LH and estradiol surges and suppression of progesterone for more than 6 months. The return of menstrual cyclicity preceded the return of ovulatory cyclicity. Menstrual cyclicity occurred while the drug was still detectable in the blood. The probable site of action of the drug is discussed.