Physiological role of the adrenal medulla was evaluated in the rat with special reference to the age. The adrenal medulla of Wistar-Imamichi male and female rats was enucleated (AdMx) or sham-operated at 3 or 12 weeks of age. Blood pressure was determined by the tail-cuff method. Systolic blood pressure in immature- and adult-AdMx rats increased progressively 5 approximately 6 weeks after the operation. Plasma renin activities in the AdMx male and female rats tended to be lower than those in controls. There was no significant difference in the pressor response to 50 micrograms/kg l-adrenaline (s.c.) between control and AdMx groups. Isoproterenol induced a greater decrease in blood pressure in immature AdMx rats. Administration of 200 micrograms/kg tyramine resulted in a marked increase in blood pressure in the immature AdMx males and in a less marked response in adult AdMx and sham-operated rats. In 24 hr-reserpinized immature AdMx male rats, blood pressure was maintained at approximately 110 mmHg, and tyramine induced a slight increase in the blood pressure. In immaure control male rats, blood pressure showed a decrease to 80 mmHg 24 hr after reserpinization, and no response to tyramine was evident. The results indicated that the lack of an adrenal medulla from the prepuberal age, particularly in male rats, develops alterations in the regulatory system of peripheral noradrenaline release.