It has now been established that there is an interaction between the cardiovascular control systems and opiate substances. To assess the role of endogenous opiates in the long-term control of blood pressure, the opiate antagonist naloxone (0.5 mg/kg/24 h) was infused intravenously for 7 days. Chronic blockade of endogenous opiates resulted in an elevation in arterial blood pressure which averaged 11% and was sustained throughout the infusion period. This was accompanied by a significant decrease in plasma renin activity averaging -59%. These data provide support for a tonic role of opioids in the long-term control of the cardiovascular system which may involve baroreceptor function.