LH levels are inappropriately elevated in women with polycystic ovary syndrome (PCO). Regardless of whether this LH elevation is primary or secondary it may be involved in the establishment of chronic anovulation. We have therefore tried selectively to suppress LH by giving medroxyprogesterone acetate (MPA) 5 mg twice daily for 14 d to 13 women with PCO (14 studies) and have examined their pituitary and ovarian response to assess whether follicular development and ovulation would be triggered post-treatment as a result of secretion of endogenous gonadotrophins in a more nearly physiological ratio. Blood samples were obtained before and two to three times weekly during and after treatment for 35-50 d. In two of these cycles MPA was started fortuitously during the luteal phase of a spontaneous cycle. In the remainder, three patterns of response were seen. In 9 cycles, after an initial rise, there was a progressive fall in gonadotrophin levels, LH (mean 48% of basal) more than FSH (mean 68% of basal); after MPA was stopped FSH levels recovered more rapidly than LH. In two cycles (one subject) MPA administration was associated with a progressive rise in serum oestradiol and progesterone. In a further cycle, treatment was also accompanied by an oestradiol rise and was followed by establishment of regular ovulatory cycles. The observed differential suppressive effect on LH and FSH levels suggests that MPA might be of therapeutic value if followed by treatment with anti-oestrogens or exogenous gonadotrophins in PCO.