Number and affinity of pontine muscarinic receptors were examined in rats sacrificed during polygraphically defined wakefulness (W), synchronized sleep (S) and desynchronized sleep (D). Saturation binding assays with (-)-[3H]quinuclidinyl benzilate [( 3H]QNB) showed that muscarinic receptor levels were higher in W and D than in S. These findings indicate that brain receptor levels may change in relation to sleep-waking states. The significance of such changes in relation to the time course of sleep-waking states, as well as the possibility that the number of pontine muscarinic receptors plays a role in the triggering of D are discussed.