The hibernating female Turkish hamster (Mesocricetus brandti) was utilized for a study of possible in vivo effects of cold on oocyte maturation. Such a physiologic model offered an opportunity to analyze the ability of oocytes exposed to prolonged periods of reduced core temperature and/or light to subsequently mature to Metaphase II. Detailed observations of core temperatures, torpor/arousal, serum estradiol, and ovarian histology were made. An average incidence of 37.7% binuclearity was found in the germinal vesicle, metaphase I and II occytes of this species. Maturation to Metaphase II of total chromosome complements did not vary significantly in the experimental groups compared with the control, but aneuploidy was detected in the oocytes of animals exposed to reduced temperature or light. An effect of in vivo reduced core temperatures on oocyte chromosome complements validates many of the published in vitro studies with temperature reduction. The model presents an excellent physiologic system for perturbing and analyzing many aspects of mammalian oocyte development.