Plasma specimens were obtained in the afternoon (13.30 to 17.29 h) from 668 post-menopausal women and in the evening (17.30 to 20.30 h) from 219 further women. The mean evening plasma prolactin level was significantly higher than that found in the afternoon (p less than 0.0007). Mean afternoon prolactin levels were not related to nulliparity and body weight. In nulliparous obese women the mean of the evening peak prolactin concentrations was double that of comparable women studied in the afternoon (p less than 0.003).