Amounts of sleep and waking were determined in rats at ambient temperatures (Ta's) of 20, 25, and 30 degrees C, before and after basal forebrain lesions. Rats were hyposomniac at all Ta's for 1-2 days postlesion. After that, sleep was highly Ta-dependent. Rats were typically hyperthermic after complete ablation of the medial preoptic area (MPOA), and the Ta at which maximal amounts of rapid-eye movement sleep (REMS) occurred frequently shifted from 30 to 25 degrees C. During the first postlesion month, amounts of slow-wave sleep (SWS), REMS, total sleep time ( TST ), and the proportion of time spent in REMS to TST (REMS/ TST ) all improved significantly at the Ta's at which the most REMS occurred (high REMS temperatures). In contrast, at the Ta's at which the least REMS occurred, these variables were as depressed one month after MPOA damage as they were at 5 days postlesion. REMS/ TST recovered most rapidly, returning to prelesion levels at high REMS temperatures within the first postlesion week. REMS bout durations were severely shortened after forebrain damage, and this was the only sleep disturbance not attenuated at high REMS temperatures. After smaller basal forebrain lesions, initial deficits were less severe and normal amounts of sleep returned earlier. However, as was the case for large lesions, sleep deficits were most severe and persistent at low REMS temperatures.