Stimulation of rat pinealocyte cyclic GMP by beta-adrenergic agonists (in the presence of prazosin to block alpha 1-adrenoceptors) showed an identical potency order (isoproterenol greater than epinephrine = norepinephrine much greater than salbutamol greater than terbutaline) to the stimulation of pinealocyte cyclic AMP. The increase in cyclic AMP and cyclic GMP produced by norepinephrine was inhibited by various subtype-selective beta-adrenergic antagonists with the same potency order (propranolol greater than atenolol = ICI 118,551 greater than practolol). These results indicate that pinealocytes have a single beta-adrenoceptor, of the beta 1-subtype. Activation of this beta 1-adrenoceptor mediates adrenergic stimulation of not only cyclic AMP but also cyclic GMP.