Dialysis of cerebral venous blood was performed on rabbits during sleep induced by electrical stimulation of the hypnogenic thalamic system or during sustained wakefulness elicited by stimulation of the activating reticular system. The injection of dialysate from a sleeping donor elicited in a normal recipient a moderate sleep which did not differ from normal sleep according to the behavior of the animal and the pattern of the electroencephalogram. The action of the dialysate obtained from sleeping donors was significantly different from that obtained from control donors after sham-stimulation of the thalamus. Injection of dialysate from alert donors elicited a behavioral and electrographic arousal with rhinodiencephalic activation symptoms. These results suggest that dialyzable humoral factors play a role in the mediation of sleep and wakeleness.