Ambient temperature related sleep changes in rats neonatally treated with capsaicin. PHYSIOL BEHAV 00(0) 000-000, 2004. The study was conducted on adult male Wistar rats, neonatally treated with capsaicin to destroy the peripheral warm receptors. The sleep-wakefulness was recorded for 5 h at an ambient temperature (T amb ) of 18, 24, 30 and 33 °C on different days. The rectal temperatures (T r ) of the rats were studied on exposure to 6 and 37 °C for 2 h to assess their thermoregulatory ability. The changes in the behavioral thermoregulation were assessed by noting the thermal preference of rats when they were placed in an environmental chamber with 3 interconnected compartments maintained at 24, 27 and 30 °C. Slow wave (SWS) and rapid eye movement (REM) sleep were decreased at 18 °C and increased at 30 °C, in control rats. There was a decrease in REM sleep and no change in SWS when T amb was raised from 30 to 33 °C. However, in neonatally capsaicin treated rats, sleep was increased even at 33 °C, though there was no significant change in sleep when T amb was increased from 18 to 24 °C. Capsaicin treated rats showed thermoregulatory deficiency at 37 °C but the thermal preference was unaltered in these rats. The results suggest that the central warm receptors can produce alteration in sleep at different T amb , even in absence of peripheral warm receptors. The behavioral thermoregulation was unaffected in these rats, though their ability to defend the body temperature in warm environment was affected.