It has been reported that naloxone, an opiate receptor antagonist, blunts the hypotensive effect of captopril in normotensives. However, our previous data did not show any interaction between captopril given acutely and naloxone (0.1 mg/kg) in hypertensives. To test whether a greater naloxone dose could interfere with the hemodynamic effect of chronically administered captopril, 12 male hypertensives were studied: Six of them had been under captopril treatment (50 mg tid) for at least 1 month, whereas the others had been drug free for the same time. Both groups randomly received a saline or naloxone (0.2 mg/kg) infusion for 1 hour, and blood pressure, heart rate, PRA, plasma aldosterone, adrenaline, and noradrenaline were measured at regular intervals before, during, and after naloxone infusion. In drug-free hypertensives, naloxone tended to reduce blood pressure slightly and did not modify heart rate, PRA, plasma aldosterone, adrenaline, or noradrenaline. In captopril-treated hypertensives, naloxone did not blunt the hypotensive effect of captopril, but rather enhanced it, without changing the heart rate, adrenaline, and noradrenaline. Moreover, naloxone increased the renin-stimulating action and did not modify the aldosterone-inhibiting effect of captopril. Our results show that the hemodynamic action of captopril given chronically is not influenced by opioid receptor blockade and therefore that the antihypertensive effect of this drug seems to be unrelated to the activation of the opioidergic system.