Luteinizing hormone (LH) and progesterone (Pg) levels in blood were measured simultaneously by radioimmunoassay during 53 menstrual cycles in order to investigate the effect of 400 μg of D norgestrel, administered postcoitally, on pituitary and ovarian function. Of 31 control cycles, 2 appeared to be anovulatory, since the LH peak and subsequent Pg elevation were absent. Twenty nine cycles showed typical LH surges in the middle of the cycle, followed by a manifold Pg increase. Twelve women received 5 to 13 tablets of D norgestrel. A total absence or at least marked suppression of LH and Pg elevations was observed. In a third group, D norgestrel was administered on scheduled days. Each woman ingested one to four tablets between the 6th and 18th days of the cycle. Two or more tablets disturbed LH and Pg ovulatory patterns. Of the four women who received a tablet on day 10, one failed to show the characteristic ovulatory patterns and three exhibited a delay in the time of the LH and Pg increase. These results demonstrate that D norgestrel in a postcoital regimen alters pituitary and ovarian function, strongly suggesting an antiovulatory effect.