We investigated the effects of castration and testosterone propionate on tyrosine hydroxylase mRNA, its activity, and catecholamine synthesis in the adrenal medulla of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Four-week-old male rats were castrated. Testosterone propionate (500 micrograms per rat) was administered subcutaneously twice a week to castrated rats (between 14 and 25 weeks of age). Systolic pressure was measured at the age of 25 weeks, and rats were decapitated. The systolic pressure of castrated SHR was significantly lower than that of control and testosterone-replaced SHR. Epinephrine and norepinephrine levels, tyrosine hydroxylase activity, and tyrosine hydroxylase mRNA in the adrenal medulla of castrated SHR were significantly lower than in control and testosterone-replaced SHR. Systolic pressure and epinephrine and norepinephrine levels, tyrosine hydroxylase activity, and tyrosine hydroxylase mRNA levels in the adrenal medulla of WKY showed no significant differences among the control, castrated, and testosterone-replaced groups. These results suggest that androgens contribute to the development and maintenance of hypertension in SHR via sustained enhancement of tyrosine hydroxylase synthesis in the adrenal medulla, leading to increased epinephrine and norepinephrine levels.