The functional development of cardiac and adrenal medullary responses to reflex activation of the sympathetic nervous system was studied in preweanling spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rat pups. Pups from the two strains received injections of insulin or saline at 2, 4, 8, 12 or 16 days of age and were killed 3 h later. The induction of ornithine decarboxylase (ODC) in the heart and the depletion of epinephrine (EPI) from the adrenal medulla served as tissue markers of functional sympathetic neurotransmission. Compared with WKY pups, SHR pups showed a greater induction of ODC activity in the heart at 2, 4, and 8 days of age. There were no differences between SHR and WKY pups in the adrenal medullary response to insulin-induced hypoglycaemia. These altered patterns of sympathetic-target tissue development in early life may contribute to the higher arterial pressures maintained by SHR throughout life.