According to monoaminergic theories of rapid-eye-movement (REM) sleep, phentolamine (PHEN), an alpha-adrenoceptor antagonist that diminishes noradrenergic activity, should decrease REM sleep. However, PHEN also causes a fall in body temperature (Tb) that depends on the ambient temperature (Ta). When REM sleep and Tb were recorded in the same rats at 4 Ta's, after intraperitoneal injections of either PHEN or saline, the two measures were highly correlated. When PHEN lowered Tb, at Ta 20 degrees C, latency to REM sleep increased and total REM time decreased compared to controls. At Ta 32 degrees C, where PHEN had no effect on Tb, drug-treated rats had as much REM sleep as did controls.