Warm sensitive neurons (WSN) play a major role not only in body temperature regulation, but also in sleep regulation. The present study was undertaken to investigate the role of WSN of the preoptic area (POA) in mediating the ambient temperature (T(amb)) related changes in sleep. The effect of T(amb) changes on sleep and body temperature was studied in rats before and after destruction of WSN of the POA by local intracerebral injection of capsaicin. Though the rats preferred 27 degrees C T(amb), they slept maximum at 30 degrees C. After destruction of WSN of the POA, slow wave sleep (SWS) peak was brought down to 27 degrees C, which was the preferred T(amb) of the rats. This indicates that WSN of the POA mediate the increase in SWS, at temperatures higher than preferred T(amb). On the other hand, in WSN destroyed rats, rapid eye movement (REM) sleep was maximum at 33 degrees C. It suggests that the REM sleep generation is under inhibitory control of the WSN of the POA. The study supports several earlier reports that the neurons of the POA play a key role in coordinating sleep and body temperature regulation.