Environmental effects on ovulation and embryogenesis in Rana pipiens were assessed using both freshly-captured fall animals and laboratory-conditioned females which had undergone vitellogenesis in the laboratory. Frogs in both categories were divided into two groups. Ovulation was hormonally induced in one group of females prior to cold exposure and in the second group of animals following an 8-week-period at 4 degrees C with an 8L 16D photoperiod. The incidence of both ovulation and normal embryonic development was increased following exposure of the animals to low temperatures and short daylength. Those animals which only partially ovulated prior to cold treatment did not respond to hormone injections following the period of cold exposure. Examination of the ovaries of these females revealed a much greater degree of oocyte resorption than was found in frogs whose initial ovulation was induced only after exposure to cold temperatures. The administration of ovulation-inducing hormones prior to artificial hibernation may thus have initiated a phase of oocyte resorption which progressed even at 4 degrees C. The incidence of ovulation was similar in wild-caught and laboratory-conditioned females, but eggs from the latter showed a much lower percentage of development to Shumway stage 20. This effect may have been related to differences in the environmental factors to whcih the two groups were exposed during oogenesis.