The effect of isoproterenol, a β-adrenergic agonist, on the parotid gland of guinea pig was studied in vivo. Male guinea pigs were fasted 12 h, and then injected intraperitoneally with isoproterenol (30 mg/kg). The parotid glands were processed for light and transmission electron microscopy using conventional techniques. Morphological analysis showed massive granular secretion 2 h after isoproterenol injection and an enlarged apical surface of the plasma membrane, as indicated by the presence of microvilli. Twenty-four hours after injection of isoproterenol the gland had still not returned to its pre-stimulation state. The effect of isoproterenol decreased with time and the apical surface of the plasma membrane eventually resumed its normal (basal) appearance. These data showed that the maximun secretagogue effect of isoproterenol on parotid gland of guinea pig was evident 2 h after injection and decreased according to the time.