Seasonal and hormonal influences regulating oocyte maturation (germinal vesicle breakdown, GVBD) in ovarian follicles of Rana dybowskii were investigated. During the early winter (Dec.-Jan.) GVBD occurred at a low incidence following in vitro culture of intact follicles. Addition of progesterone of frog pituitary homogenate (FPH) to such follicles induced oocyte maturation, whereas IBMX or forskolin inhibited hormone-induced oocyte maturation. The time course of spontaneous in vitro maturation varied markedly with the seasons and between animals. Follicles isolated from the ovaries in early February required 21-24 hours of culture to mature spontaneously, and addition of FPH or progesterone to the culture medium markedly accelerated the time course of GVBD. In contrast, follicles isolated in late February matured very rapidly (less than 6 hours), and FPH or progesterone were ineffective in accelerating the time course of GVBD. IBMX and forskolin separately or in combination stimulated follicular progesterone production, which resembled that seen following FPH stimulation. FPH addition to such follicles shifted the steroid peak to the left (accelerated) and increased the absolute amount of hormone detected in late-maturing follicles (50% GVBD, about 18 hours) but not in rapidly maturing follicles (50% GVBD, 3 hours). In contrast to other amphibians, a high incidence of spontaneous oocyte maturation occurred during in vitro culture. Essentially all animals exhibited spontaneous maturation during the normal breeding season, even those animals collected in the early winter and kept in artificial hibernation at 4 degrees C for extended periods.