The regulation by alpha- and beta-adrenergic receptor agonists of rat pineal N-acetyltransferase (NAT) activity during development was studied. We found that isoproterenol, a beta-adrenergic agonist, exhibited a weak stimulatory effect on pineal NAT activity during the first 3 weeks of age. Indeed, the drug did not mimic the effect of darkness on pineal NAT activity until the fourth week of age. The results might be explained by the existence of other mechanisms, in addition to those that involve beta-adrenergic receptors, which regulate pineal NAT activity during early postnatal development. In line with this, we determined that phenylephrine, an alpha 1-adrenergic agonist, stimulated NAT activity during both the day and at night in 2-week-old rats, while exhibiting no effect in adult rats. Methoxamine, an alpha 1-adrenergic agonist more selective than phenylephrine, also activated NAT activity in young, but not in adult, rats. Moreover, prazosin, a selective alpha 1-adrenergic receptor blocker, prevented pineal NAT activation by norepinephrine or darkness at night in 2-week-old rats, but not in adult animals. The results suggest that alpha 1-adrenergic receptors may be more important than beta-adrenergic receptors in regulating rat pineal NAT activity during development.