Animal environment is affected by climatic factors that include temperature, humidity, radiation, and wind. Extremes in climate alter energy transfer between the animal and its environment and can affect deleteriously reproduction. Seasonal variation of environment, nutrition, and management alters estrous activity and duration of estrus. Conception rates are reduced under stress of heat and cold. Endocrine functions are altered by climatic extremes. In hyperthermia, adrenal function is reduced, and this may allow the animal to cope with the environment because of the lower calorigenic actions of glucocorticoids. Estrogens are lower during the proestrus to metestrus period of the estrous cycle and during late gestation and appear to manifest their physiological actions through shorter duration of estrus and lower calf birth weights, respectively. Season alters endocrine profiles and influences fertility of males. Spermatogenesis is impaired, and testosterone is lower during early exposure to hyperthermia. Environmental modifications can alleviate stress of heat and cold to some extent. Experimentation using indices of environmental measures is needed to assess interactive effects of environment on reproduction.