Oxygen consumption, cutaneous blood flow, and length of "quiet sleep" periods were measured in 5 premature infants with birth weights ranging from 1200 to 2210 g during 15 experimental periods of 2 to 4 hours each. The unclothed infants were exposed to environmental temperatures of 32 to 34oC (50% relative humidity) during the studies. The rectal temperatures of larger infants (body weights above 1500 g) were found to be between 36.5[degree]C and 37.5[degree]C (Group 1), and those of the smaller infants were between 35 and 36.5[degree]C (Group 2). In spite of the lower body temperatures, the lengths of quiet sleep periods were no shorter in the infants of Group 2, which was considered evidence that they were not thermally "discomforted". The infants with lower rectal temperatures also did not show a thermoregulatory increase of O2 consumption, and skin blood flow in the heel increased while they were at a temperature of 32 to 34[degree]C. Exposing them to environmental temperatures below 32 to 34[degree]C, however, evoked an increase in oxygen consumption, shortening of the total length of quiet sleep, and vaso-constriction. It is concluded from these studies and previous ones that temperatures of 32 to 34[degree]C can be considered as the lower limit of the neutral temperature range, which is also the range of "thermal comfort".