A combination of 0.5 mg levonorgestrel (in 1 mg dl-norgestrel) and 0.1 mg ethinylestradiol was administered to eight volunteers 48 hours after the start of the luteinizing hormone surge. A second dose was given 12 hours later. Endometrial samples were obtained 24 hours after the first dose was given. The steroid receptor concentration was compared with ovulatory spontaneous cycles. The dl-norgestrel/ethinylestradiol combination caused a significant reduction in receptor concentration. Isocitrate dehydrogenase (a progestin-sensitive enzyme) was also altered, suggesting an effect on endometrial metabolism. Danazol was used in a similar fashion, with two doses each of 400 mg. Nine volunteers were studied. A similar pattern of alteration of endometrial biochemistry was demonstrated but did not reach statistical significance. The relevance to the postcoital use of hormones is discussed.