Bilateral or unilateral, localized and reversible cooling (+10 degrees C) of the anterior part of the locus coeruleus area (LCp) in the cat induced, in rapid succession, slow wave sleep (SWS) and a transitory phase of SWS + PGO. These states of sleep were followed in 40-50% of the animals by paradoxical sleep (PS). These effects (on SWS, SWS + PGO and PS) occurred following repetitive periods of short duration cooling and were maintained during long duration cooling (70 min). Unilateral cooling of the ventral part of the locus coeruleus area (LC alpha) and peri-LC alpha induced wakefulness, regardless of the states of sleep in which the cooling was performed. After a cryolesion (-24 degrees C) of the nucleus raphe dorsalis, cooling of the LCp did not induce a hypnogenic effect. Electrical stimulation of the locus coeruleus area, however, always induced wakefulness.