To examine the effect of night shift on the ovarian function, 122 teachers, 67 office workers, 377 nurses, 133 factory workers and 67 barmaids were surveyed. The incidence of irregular menstrual cycle was 13.1% in teachers, 14.9% in office workers, 24.9% in nurses, 36.8% in factory workers and 40.3% in barmaids. The incidence was significantly higher in women working at night than women working during the day. Plasma concentrations of melatonin, LH, FSH and prolactin were determined at 2200 h and 0200 h in 5 nurses working at night and in 6 nurses resting in their quarters. Plasma concentrations of melatonin and prolactin at 0200 h were significantly lower in nurses of the working group than others of the resting group, but plasma concentrations of LH and FSH did not differ between the two groups. These results indicate that night shift suppresses the ovarian function by affecting the circadian rhythm of melatonin and prolactin.