We assessed various aspects of endocrine function in patients treated for essential hypertension with manidipine chloride, a calcium channel blocker, to study the effects of this drug on several endogenous humoral factors, including human atrial natriuretic peptide (hANP) and endothelin 1 (ET), which influence vasoconstriction and blood fluid volume. The study included 19 patients with essential hypertension. All patients received manidipine chloride 10 mg/d for 24 weeks. After treatment blood pressure was normalized in all patients, and there were no significant changes in plasma renin activity or blood concentrations of vasopressin, hANP, ET, aldosterone, adrenaline, or noradrenaline. There was a statistically significant negative correlation between the change in ET levels and hANP levels before and after treatment. We also observed a statistically significant negative correlation between the change in systolic blood pressure caused by manidipine chloride and the change in hANP levels before and after treatment. These findings suggest that hANP and ET levels are systematically changed by manidipine chloride in patients with essential hypertension. We also speculate that changes in blood pressure may be closely related to levels of hANP in patients treated with manidipine chloride.