Phentolamine, an alpha-adrenoceptor antagonist, both decreases rapid-eye-movement sleep (REMS) and causes a dose- and ambient temperature (Ta)-dependent drop in body temperature (Tb). The purpose of this paper was to examine the correlation between changes in sleep and changes in Tb after phentolamine at various Ta's. Tb and sleep were recorded in male rats for 4 h after i.p. injection of either saline or phentolamine (1 mg/kg at Ta 20 degrees C; 5 mg/kg at 20, 30 and 32 degrees C; 10 mg/kg at 20, 30, 32, and 34 degrees C). Changes in sleep were highly correlated with changes in Tb: when Tb dropped, amounts of sleep, especially REMS, also were decreased. The largest effects were seen at Ta 20 degrees C. After 5 mg/kg, Tb was below normal for 2 h and latency to REMS increased 82 +/- 27 min (P less than 0.025). After 10 mg/kg Tb was decreased for 3 h, and latency to REMS increased 91 +/- 19 min (P less than 0.001). While Tb was lower amounts of REMS were also decreased. Smaller effects were observed on slow wave sleep. At those conditions where phentolamine had no effect on Tb (i.e. Ta 20 degrees C, 1 mg/kg, and Ta 32 degrees C, 5 and 10 mg/kg) no differences were found in any measure of sleep. These results demonstrate that the effects of phentolamine on sleep are not caused by direct action on sleep mechanisms, but rather by its actions on thermoregulatory and possibly other mechanisms that modulate sleep.