Melatonin levels were determined in plasma samples obtained at 15 minute intervals during a 4-hour period (08:00 - 12:00 hours) from a normal adult male on 5 consecutive days. On days 1, 2, and 3, the subject was given estradiol valerate (E2) 10 micrograms/kg at the end of each sampling period. An episodic pattern of melatonin secretion was found. Post E2 mean estradiol levels per 4 hours increased and mean melatonin and testosterone levels per 4 hours decreased significantly. A decrease in melatonin levels post E2 is the reverse of the response expected based on nonhuman animal experimental data.