1. Captopril (100 mg/kg, orally) decreased blood pressure in spontaneously hypertensive (SH) rats.
2. Bilateral adrenalectomy either before or after captopril administration did not alter the antihypertensive effect of captopril.
3. Bilateral nephrectomy reversed the established antihypertensive effect of captopril and prevented any change in blood pressure to a subsequent dose of captopril.
4. It is concluded that kidneys but not adrenal glands are essential to the antihypertensive actions of captopril in SH rats.