Adult female rhesus monkeys exhibiting normal ovulatory menstrual cycles were treated with progesterone nasal sprays. Animals in group A (n = 9) were treated with the solvent only (controls). Animals in groups B (n = 6), C (n = 17) and D (n = 7), respectively, were treated with a daily dose of 0.4, 2 and 10 micrograms of progesterone and the spraying was done between days 5-14 of the cycle. Ovulation was monitored by laparoscopy on day 20. The serum endocrine profile throughout the treated menstrual cycle was studied with respect to oestradiol and progesterone. Bioactive luteinizing hormone (bLH) was studied in blood samples taken on the day of the mid-cycle oestradiol peak, 2 days before, and 2 days after. The menstrual cycle was divided into two phases with respect to the mid-cycle oestradiol peak: phase I was taken to include day 1 of the cycle to the day of the oestradiol peak, and the remaining part of the menstrual cycle was considered to be phase II. The serum-endocrine profile in the controls was similar to that observed in normal ovulatory menstrual cycles. However, in the progesterone-treated groups three types of menstrual cycles were discernable on the basis of the serum endocrine profile. In the type I menstrual cycle, observed only in group C (n = 10), the mid-cycle bLH peak was abolished and the progesterone levels remained low throughout the cycle. Laparoscopy revealed these to be anovulatory cycles.(ABSTRACT TRUNCATED AT 250 WORDS)