SummaryImmunoreactive human serum prolactin of pituitary origin has been measured by a radioimmunoassay developed for ovine prolactin. Blood samples were coilected at four-hour intervals during a 24-hour period from 12 non-pregnant women, three pregnant women, and seven adult men. A circadian periodicity was found in serum prolactin concentration, with peak values during the night, between 1 a.m. and 5 a.m. for the nonpregnant women, and at 5 a.m. for the adult men. Mean serum levels of prolactin were 1'5 times higher in nonpregnant women than in men. In women investigated during the last month of their pregnancy the mean serum prolactin levels were 2'3 times higher than in the non-pregnant women, but there was no circadian periodicity. Introduction A circadian periodicity in serum prolactin concentration has been found in rats (Clark and Baker, 1964;Dunn et al., 1972). The purpose of the present study was to investigate whether a similar rhythm exists in man.

14of prolactin in all clinical and physiological situations so far investigated. However, routine establishment of the technique still requires that larger supplies of purified human prolactin are made available.