Spontaneously hypertensive rats (SHR) of the Okamoto-Aoki strain (n = 40) were treated with captopril (SQ 14,225; D-3-mercapto-2-methylpropanoyl-L-proline) orally, dose 0.2 mg/ml in drinking water. The treatment was initiated early and later during the course of developing hypertension. Continuously treated rats did not develop hypertension. Rats receiving captopril for 12 weeks remained normotensive, whereas withdrawal of the drug resulted in hypertension. Captopril treatment was effective in the rats with established hypertension and decreased the blood pressures to nearly normal values. Serum angiotensin converting enzyme (ACE) activity rose 3-fold in captopril treated rats. ACE in lung plasma membranes increased during captopril treatment, indicating that captopril induced biosynthesis of pulmonary ACE. No qualitative differences were found in the ACE from treated and not treated animals. The dissociation of the antihypertensive effect of captopril and of increased ACE activity in serum and lungs reduce the value of relating blood pressure effects of the drug to measured enzyme activity in the SHR.