To investigate the role of norepinephrine release from the sympathetic nerve endings and vascular responsiveness in the pathogenesis of hypertension, the perfused mesenteric preparations were used in DOCA-salt hypertensive rats (acute phase: 10 days after operation, chronic phase: 7-8 weeks). In addition, the effects of a Ca-antagonist (verapamil) on the norepinephrine release and vascular responsiveness were also examined. Vasoconstrictor responses to the electrical nerve stimulation were significantly greater in DOCA-salt hypertension in the chronic phase than the age-matched normotensive controls. The pressor responses to exogenous norepinephrine were significantly enhanced in DOCA-salt hypertension both in acute and chronic phases. Endogenous norepinephrine overflow from the sympathetic nerve endings during the electrical nerve stimulation was enhanced in the chronic phase of DOCA-salt hypertension, but not in the acute phase, compared with the age-matched normotensive controls. After infusion of verapamil, the pressor responses and norepinephrine overflow by the electrical nerve stimulation were significantly inhibited, and the suppression was greater in chronic DOCA-salt hypertension than in the normotensive controls. These results demonstrate that the vascular responsiveness was increased in both acute and chronic phases of DOCA-salt hypertensive rats, while the norepinephrine overflow from the adrenergic nerve terminals was enhanced only in the chronic phase. More marked inhibition of the vasoconstrictor responses and norepinephrine overflow in the presence of a Ca-antagonist in chronic DOCA-salt hypertension might represent the higher Ca-dependency in the neurotransmission of the peripheral resistance vessels, especially in the mechanism of presynaptic norepinephrine release, than their normotensive controls, and it could partly contribute to the development and maintenance of DOCA-salt hypertension.