Adult male and female spontaneously hypertensive rats (SHR-Okamoto, Kyoto) were gonadectomized when they were 17 weeks old. Intact SHR showed a progressive increase of their blood pressure with growth, attaining systolic pressure levels of 194-208 in males and 163-173 mmHg in females when they were 29-30 weeks old. During this same period, the gonadectomized animals showed a significant reduction in blood pressure ranging from 168-175 in males and from 158-163 mmHg in females. These studies indicate that male and female SHR gonadectomized at 17 weeks of age do not show the progressive blood pressure rise that occurs in intact SHR. There was no change in heart rate in either sex. Corticosterone (B) levels in plasma were increased in the orchidectomized males, and 18-OH-DOC levels in plasma were increased in the adrenal glands of ovariectomized females indicating that these hormonal changes probably do not play a role in SHR hypertension. It appears that gonadal and other hormones are involved in the pathogenesis of SHR hypertension.