The present findings show that while chronic administration of the catecholamine isoproterenol to immature rats causes accelerated structural development, it causes an inhibition of certain aspects of secretory function. The effects on both structure and function are however closely dependent on the stage of development during which the agent is administered. When the isoproterenol is given for 8 days between 12 and 20 days of age, amylase, Na, and K concentrations of pilocarpine-evoked parotid saliva of the treated and control littermates are similar. If, however, the agent is given between 22 and 30 days of age, amylase of the isoproterenol-treated glands is only one-fourth that of controls, and K concentration is higher than that of controls whereas Na is lower. The glands of controls in both pre- and postweanling groups produce greater volumes of water than do isoproterenol-treated glands. Developmentally, these data represent less advanced secretory abilities, and therefore isoproterenol induces inhibition of secretory capacity, rather enhancement.