Four days of isoproterenol injections induced a marked enlargement of the rat parotid and submandibular glands reflected in significant increases in the absolute and relative wet and dry weight of the glands. The enlargement in parotid gland was attributable at least in part to cellular hypertrophy inasmuch as the average volume per cell of acinar cells increased. In contrast, the average volume of acinar cells in the submandibular gland was decreased as compared to that of control. It is likely that hyperplasia in both groups accounts in part for the enlargement. The slow calcium channel is unlikely involved in the isoproterenol-induced stimulation of the gland, inasmuch as the calcium channel antagonist did not modify the enlargement of the parotid or submandibular glands.