There is evidence that adenosine 3′,5′-monophosphate (cyclic AMP) serves as a mediator of the secretion of several hormones (Bowers, 1971) as well as a 'second messenger' in hormone action (Sutherland, Robison & Butcher, 1968). However, information on the effect of cyclic AMP on the secretory mechanism of prolactin is scanty. Lemay & Labrie (1972) found that N6-monobutyryl adenosine 3′,5′-monophosphate caused increased release of pituitary prolactin as measured by densitometry of acrylamide gels. We have confirmed their results by radioimmunoassay using N6, O2′-dibutyryl adenosine 3′,5′-monophosphate (dibutyryl cyclic AMP) and further have demonstrated that dibutyryl cyclic AMP acts primarily on the release of prolactin and not on its synthesis. Each anterior pituitary obtained from 10-week-old Wistar—Imamichi male rats (230–250 g) was placed in 1 ml Medium 199 (Difco Labs., Detroit, Michigan). Incubation was carried out in a Dubnoff metabolic shaker under constant gassing with 95% O2