The purpose of this study was to test the hypothesis that fluctuations in progesterone levels can account for the psychophysiological changes occurring during the luteal phase of the menstrual cycle. In order to avoid the confounding of endogenous and exogenously administered progesterone as might occur in women, we gave progesterone to normal men and studied the time course of its effects under controlled conditions.Progesterone (10 mg/day) was administered daily to six men during either the second or third week (double blind) of a 4− to 4$$-week period. On 4 days per week a 20− to 25-minute testing procedure was carried out in which skin conductance (SC), heart rate (HR), and respiration were recorded continuously during rest, presentation of mild tones, and performance of time estimation and reaction time tasks. Body temperature was taken before arising in the morning and a mood scale was given daily. Results showed a significant rise in temperature and reaction time and decrease in HR variability during progesterone administration which returned to baseline soon after stopping drug intake. Significant decreases in SC level and increases in the “sluggishness” of spontaneous and elicited SC responses persisted for a week postdrug. Expected increases in base HR and respiration rate and changes in mood were not observed to a significant degree. The data only partially support the progesterone hypothesis of premenstrual tension and suggest that the role of estrogen or some other hormone may be important. It is suggested that differential recovery of different physiological symptoms may be related to the psychological symptoms following progesterone withdrawal.