Rat ovarian follicles were isolated at nine different times during proestrus and cultured for 3 d. During the period of cultivation, estrogen and progestagen secretion was measured. Other follicles were dissected out at the same time periods and analyzed directly for these hormones. The most intense hormone release was observed during the first 24 h in culture. The concentration of steroids secreted in vitro by follicles previously collected on the morning of proestrus was relatively low. The secreted estrogen concentration increased gradually from follicles previously isolated at later times during proestrus, with a maximum at 1800. The peak secretion of progestagens in culture was from follicles isolated at 2000. There was a marked decrease in secretion of both hormones from follicles isolated past their optimal times. The same secretion pattern of estrogens and progestagens was shown by follicles in vitro at 24 h as with similar follicles in vivo, according to their time of isolation. From these results it is evident that the normal rhythm of steroid secretion, predetermined at the time of follicle excision, was maintained during 24 h in organ culture. After a longer period in culture, the dynamics of hormone synthesis became altered, although follicles appeared morphologically normal up to 48 h.