The push-pull cannula was used to examine the release of endogenous norepinephrine (NE) from the paraventricular hypothalamic nucleus (PVH) of unanesthetized freely moving 7- to 10- and 12- to 14-wk-old Sprague-Dawley (SD), Wistar-Kyoto (WKY), and spontaneously hypertensive (SHR) rats. Basal NE release, K+ (56 mM) stimulation-induced NE release, and NE release in response to pressor/depressor drugs were examined in all three strains at both ages. Significant increases in basal and K+-stimulated release of endogenous NE from the PVH were observed in 7- to 10- and 12- to 14-wk-old SHR compared with the normotensive control rats suggesting that enhanced central noradrenergic nerve activity may be involved in the development and maintenance of hypertension in the SHR. In addition, a reciprocal relationship exists with respect to blood pressure and NE release from the PVH, i.e., decreases in blood pressure elicit increases in NE release, and increases in blood pressure elicit decreases in NE release in all three strains at both age groups, suggesting that the noradrenergic pathways of the PVH contribute to the maintenance of arterial blood pressure homeostasis.