We compared the overflow of endogenous norepinephrine during electrical field stimulation, the norepinephrine content, and the rate of initial neuronal uptake of [3H]norepinephrine in synaptosomes isolated from hypothalamus and brainstem of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 7 and 13 weeks of age. The synaptosomes of two rats, a SHR and a WKY rat control, were simultaneously processed and subjected to the same electrical field stimulation. The overflow of endogenous norepinephrine during electrical stimulation (2 Hz, 2 minutes) in the hypothalamic synaptosomes of 7-week-old SHR was significantly greater, whereas the overflow of 13-week-old SHR was equivalent to the age-matched WKY rat. The norepinephrine content of synaptosomes was about the same in SHR and age-matched controls. There was also significantly enhanced [3H]norepinephrine uptake in the hypothalamic synaptosomes of young SHR, but neither the hypothalamic nor the brainstem samples of 13-week-old SHR showed any significant difference in their rate of [3H]norepinephrine uptake. These data are similar to those we observed (unpublished observations) in perfused mesenteric artery system in which norepinephrine release was significantly elevated during periarterial nerve stimulation only in young SHR. Thus, these results suggest that a parallel enhancement of norepinephrine release in hypothalamus with that of peripheral nervous system may play an important role during development of hypertension in young SHR.