Abstract: Delta sleep-inducing peptide (DSIP) has been isolated and characterized by its capacity to enhance delta sleep in rabbits. Up to now, sleep was the main target of DSIP research, but different extra-sleep effects of the peptide have been reported as well. Several mechanisms of action have been proposed, though no convincing evidence for any of them has been obtained so far. We recently detected that DSIP reduced the nocturnal increase of N-acetyltransferase (NAT) activity in rat pineal in a dose-dependent manner. The activity of this enzyme is known to be induced by adrenergic agonists and several studies have suggested that stimulation of α1-adrenergic receptors potentiates the “basic” effect of β-receptors. DSIP in the range between 20 and 300 nM significantly enhanced NAT activity induced by 10-6M norepinephrine in vitro, and a similar effect was observed with 2 nM P-DSIP, a phosphorylated analog. Incubation with prazosin eliminated the enhancement, whereas propranolol reduced norepinephrine stimulation that was still increased by P-DSIP and probably DSIP. It was concluded that the sleep-peptide and its analog modulate the α1-adrenergic receptor of rat pineal in its response to adrenergic agonists. The same mechanism may also be responsible for other biological activities of DSIP such as sleep-induction and stress-tolerance.