Thermoregulatory responses were examined in laboratory-reared, F₂ neonatal, hispid cotton rats of lineages 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 (Tb), O₂ consumption, and total conductance were noted in 1-18-day-old rats exposed, individually for 3.5 h, to ambient temperatures (Ta's) of 0-35 C. No differences were found between the thermoregulatory responses of Kansas and Texas neonates. Final Tb in 1-3-day-old neonates is equal to Ta at Ta's below 15 C. In the 15-30 C range of Ta's, Tb's are approximately 2-10 C above Ta. At Ta = 35 C, Tb is 0-2 C higher than Ta. Total conductance is greater in 1-3-day-olds than at any other age due to inadequate insulatory properties. Oxygen consumption is lower at low Ta's and increases as Ta approaches 30 C. Oxygen consumption decreases at Ta's above 30 C. After day 4, neonates are capable of homeothermy at Ta's above 20 C. Between days 5 and 13, a progressive increase occurs in oxygen consumption and capability of maintaining homeothermy at lower Ta's. Total conductance decreases at all Ta's by the thirteenth day of life. From days 14 to 18, O₂ consumption and total conductance decrease at Ta's below 35 C. Capability of individuals to thermoregulate appears to be more closely related to age than to body mass. Onset of visible shivering was noted in all neonates by days 4-7. Litters of four neonates, enclosed in a cotton nest and exposed for 3.5 h to 0 C, maintained homeothermic Tb's by the third day of life.