Thermoregulatory abilities were examined in laboratory-reared neonatal and adult hispid cotton rats, Sigmodon hispidus texianus, from northern Kansas and south-central Texas. These regions are representative of the northern and southern areas of the distribution of the subspecies;Rectal body temperature (T(,b)), O(,2) consumption and total conductance were noted in 1- to 18-day-old rats exposed, individually, to ambient temperatures (T(,a)) of 0-35 C. No differences were found between the thermoregulatory responses of Kansas or Texas neonates. One- to 3-day-old neonates have T(,b)'s which are 2-10 C above T(,a). Total conductance is highest in 1- to 3-day-olds due to the sparse amount of fur. After day 4, neonates are capable of homeothermy at T(,a)'s above 20 C. Between days 5-13, a progressive increase occurs in the capability to maintain homeothermy at increasingly lower T(,a)'s. At the same time, O(,2) consumption increases at lower T(,a)'s and decreases at higher T(,a)'s. Total conductance decreases at all T(,a)'s by the 13th day of life. From days 14-18, rates of O(,2) consumption and total conductance decrease at T(,a)'s below thermoneutrality. Capability of individuals to thermoregulate appears to be more closely related to age than to body size. Litters of 4 neonates enclosed in a cotton nest and exposed to 0 C were determined to maintain homeothermic T(,b)'s by the 3rd day of life;Thermoregulatory responses of Kansas and Texas adults subjected to T(,a)'s of 0-40 C were significantly different. Kansas animals had a basal metabolic rate (BMR) of 0.83 ml O(,2)/g(.)h(.) which is 85 % of the value predicted by weight. The BMR of Texas animals is 103 % of the value predicted by weight and is significantly higher than that determined for Kansas animals. Thermoneutral zones are between 29.5-33.7 C and 33.5-35 C in Kansas and Texas animals, respectively. Total and dry conductances are significantly lower in Kansas animals. Total conductances for both Kansas (0.078 ml O(,2)/g(.)h(.)(DEGREES)C) and Texas (0.082 ml O(,2)/g(.)h(.)(DEGREES)C) animals are higher than values predicted by body weights (113 % and 115 %, respectively). Body temperature is similar (38.1 C) at T(,a)'s below 35 C in both Kansas and Texas animals. Evaporative water loss is similar at all T(,a)'s below 40 C. However, Kansas animals when exposed to 40 C, salivated profusely and spread saliva about the entire body surface. Texas animals exhibited no such behavior. The percent of endogenous heat dissipated by evaporative water loss is significantly greater in Kansas animals than in Texas animals.