A comparative study of the temperature regulation and rate of metabolism of armadillos and armadillo-like mammals (hedgehogs and hedgehog tenrecs) is reported. Armadillos are characterized by low body temperatures, low basal rates of metabolism, and high minimal thermal conductances. Strictly temperate species of armadillos are confined in distribution to South America; they have higher body temperatures than tropical species, but these groups show no noticeable difference in basal rate or in minimal conductance. Armadillos that specialize on termites as food (e.g., Tolypeutes and Priodontes) have much lower basal rates and body temperatures than other species. Hedgehogs, hedgehog tentrecs, and marsupial bandicoots also have low body temperatures, low basal rates of metabolism, and high minimal conductances. The principal reason why armadillos have low rates of metabolism is to prevent overheating in burrows, although termite-eating species may have lower basal rates than other species due to variations in the spatial and temporal availability of prey. The high minimal conductances found in these mammals prevent overheating in burrows and reflect a protective armor of either horny plates (armadillos) or spines (hedgehogs and hedgehog tenrecs). The interaction of low basal rates and high minimal conductances results in temperature differentials between the body and environment that are independent of mass and only about 25% of the values expected in mammals. Studies of daily energy expenditure in Dasypus novemcinctus show a circadial rhythm of activity and indicate that this species responds to a sharp fall in environmental temperature by a reduction in activity and a modest reduction in body temperature. Large individuals may take as long as 3 or 4 days for normal body temperature to be reestablished after such a cold exposure. Armadillos have a limited capacity to invade cold, temperate environments because food is unavailable at environmental temperatures below 0°C and because of their high thermal conductances. Armadillos or other similar mammals enter cold, temperate environments either by storing enough fat to wait for foraging to be possible again (D. novemcinctus) or by entering torpor (Zaedyus, Eri-naceus, Setifer), a response that requires a small body size. Tolerance to starvation through an increased mass is limited by the necessity to seek refuge in a burrow. Young-of-the-year, lean individuals, and pregnant females are most susceptible to starvation at the geographic and climatic edge of their distribution.