It has repeatedly been demonstrated that administration of prostaglandin (PG) F-2α to nonprimates is luteolytic and inhibits progesterone synthesis, while administration of PGE-2 stimulates progesterone synthesis. The luteolytic effect of PGF-2α has not been conclusively demonstrated in vivo in the primate, variable effects on the circulating progesterone levels having been reported (Shaikh, 1972; Auletta, Speroff & Caldwell, 1973; Shaikh & Klaiber, 1974; Korda, Shutt, Smith, Shearman & Lyneham, 1975; Lyneham, Korda, Shutt, Smith & Shearman, 1975). PGE-2 has been reported to increase progesterone synthesis by the primate corpus luteum (Channing, 1971). It is becoming apparent that route of administration, dosage and other unknown factors can alter the effect of PGF-2α on progesterone synthesis (Channing, 1972; Auletta et al., 1973; Puri & Laumas, 1975; Henderson & McNatty, 1975). Attempts to isolate the factors whereby PGs affect granulosa cells by the use of in-vitro systems have met with limited success, because PGs in vitro frequently exhibit effects which differ from those seen in vivo (Lindner et al., 1974). We therefore examined the effect of the developmental state of the follicle on its response to PG and dibutyryl cyclic AMP in vitro. Dibutyryl cyclic AMP (dbcAMP) is considered to mimic the action of gonadotrophins, because synthesis of its analogue, cAMP, is believed to be a step in the intracellular action of gonadotrophins (Marsh, 1976).