Ovarian function was assessed in 10 healthy young women, before and after the insertion of 3 or 4 polydimethylsiloxane capsules filled with 20 mg of megestrol acetate. Each capsule released in vitro, approximately 20 μg/24 h of the hormone. Daily determination of the urinary excretion of FSH, LH, fractionated oestrogens and pregnanediol were performed in all subjects during one control cycle, the first and the third cycle after the insertion of the capsules. Out of 10, 8 control cycles were ovulatory according to all the parameters investigated. This compares with 15 ovulatory cycles out of a total of 20, examined after the insertion of the capsules. During treatment no changes were observed in the FSH excretion pattern; the mid-cycle LH peak was present in all ovulatory cycles, although it was usually much less evident under the action of megestrol acetate. The excretion of oestradiol was significantly increased in all subjects (P < 0.05) during the first cycle following implantation. Oestrone and oestriol excretion was also generally higher in patients bearing PDS capsules; however, this difference was not statistically significant. Pregnanediol levels were not affected by the treatment in all cycles considered to be ovulatory on the basis of all the parameters. The menstrual bleeding pattern did not change in the majority of cases. One patient had, during treatment with 3 capsules, two profuse break-through bleedings whereas another one became amenorrhoic two months after the insertion of 4 implants. It is concluded that megestrol acetate sustained release preparations do not inhibit ovulation under the experimental conditions used.