INVESTIGATIONS were conducted with beef females to study the long-time effects of 32 C and 60% relative humidity on reproductive efficiency and adaptation, including growth rates. Physiological responses —ovarian activity, length of estrous cycle, fertility, body temperature, respiration rate, hair coat growth, body growth rate and water intake were studied. In females accustomed to cold weather (avg 1.2 C), rectal temperatures and respiration rates increased approximately 1.5 C and 90 respirations/minute, respectively, reaching a peak at 7 weeks and by the 11th week all had either plateaued at a level different from the preliminary period or had returned to normal levels. The most pronounced change observed was the shedding and decreased hair coat depth in the winter females. These females had become anestrus but had on the average reestablished their cycles by the 16th week. When these females were bred, they conceived and delivered normal calves. In females accustomed to summer weather (avg 24.4 C), body temperatures and respiration rates rose rapidly, then declined rapidly followed by marked fluctuations. The depression of gains were less than the winter conditioned females and had returned to normal by the fifth week. The hair coat continued to grow for 4 weeks and then remained stable. None of the summer condition females ceased to cycle when exposed to the 32 C heat stress. When summer conditioned females were exposed to 38 C all became anestrus.