Using a technique developed by Esler et al in man, we determined the NE spillover rate in plasma, the NE clearance and plasma NE concentrations in chloralose anesthetized control and DOCA-salt hypertensive rats. The study was undertaken with the infusion of tracer concentrations of high specific activity tritiated norepinephrine. Determination of the steady state of circulating NE specific activity provided an estimate of NE clearance and release rate. As previously reported endogenous plasma NE levels were greatly increased in DOCA-salt hypertensive rats. The NE clearance was only slightly decreased while the spillover rate was significantly increased in those animals. It can thus be concluded that the increased plasma NE levels observed in DOCA-salt animals cannot be attributed primarily to a diminished NE clearance rate but mainly to an increased diffusion from the sympathetic synaptic cleft to the plasma. Moreover, a significant linear correlation was found between the apparent release rate and the mean arterial levels whereas no correlation was found between the NE clearance and the blood pressure. Since the spillover rate is closely related to the NE release rate from sympathetic endings, this study supports the hypothesis that DOCA-salt hypertension is clearly associated with an enhanced basal sympathetic fibers activity.