The reproductive physiology of three closely related species of hamsters will be studied. Turkish, Syrian and Rumanian hamsters will be bred in the laboratory. Adult females of each species will be exposed to various photoperiodic regimens to determine the daylength required to maintain ovulatory cycles. This data will eventually be compared with data on the relationships between daylength and breeding season for each respective species in the wild. I will also explore the following areas: (1) Possible interrelationsips between photoperiod and environmental temperature in the control of cyclicity. (2) The effects of photoperiod and temperature on sexual behavor and on behavioral responsiveness to sex steroids and (3) The effects of photoperiod and temperature on pregnancy.