A suspension of (-)-Delta(9)-trans-tetrahydrocannabinol (Delta(9)-THC) (3 mg/kg body wt) was administered daily for one week by i.p. injection to female rats showing the syndrome of adrenal regeneration hypertension (ARH). Other ARH rats received no injections or injections of vehicle only. On the first day, Delta(9)-THC decreased the average blood pressure by 38+/-6 mmHg 1 h, 31+/-5 mmHg 3 h, and 11+/-5 mmHg 5 h after injection (n=0). On subsequent days the acute effect disappeared, but statistically highly significant decreases were observed between the blood pressures of the Delta(9)-THC-treated group and those of the other two groups. Plasma corticosterone concentrations, measured on the eighth day of treatment, one hour after injection time, averaged 26.+/-2.7 mug/100 ml in the untreated animals, 26.6+/-2.0 mug/100 ml in the vehicle-injected animnals, and 21.3+/-2.4 mug/100 ml in the animals injected with Delta(9)-THC. The findings indicate that Delta(9)-THC, at a moderate dose for the rat, is capable of lowering the blood pressure in rats suffering from adrenal regeneration hypertension and that chronic administration of Delta(9)-THC does not appear to stimulate the pituitary-adrenal axis, in contrast to reported effects of acute administration (Barry, Perhach & Kubena, 1970).