Unanesthetized, unrestrained kangaroo rats (Dipodomys) were studied to examine the changes in the frequency and duration of sleep states caused by long-term manipulations of hypothalamic temperature (Thy) at a thermoneutral (30 degrees C) and a low (20 degrees C) ambient temperature (Ta). A cold stimulus present in either the hypothalamus or the skin decreased both the total sleep time (TST) and the ratio of paradoxical sleep (PS) to TST. At a low Ta, TST, but not the PS-to-TST ratio, was increased by raising Thy, indicating that a cold peripheral stimulus could differentially inhibit PS. At a thermoneutral Ta, cooling Thy decreased both TST and the PS/TST. Changes in the amount of PS were due largely to changes in the frequency, but not the duration, of individual episodes of PS, suggesting that the transition to PS is partially dependent on the thermoregulatory conditions existing during slow-wave sleep (SWS). These results are consistent with the recent findings that the thermoregulatory system is functional during SWS but is inhibited or inactivated during PS.