Males are at greater risk of cardiovascular and renal disease than are females. For example, male spontaneously hypertensive rats (SHR) have higher blood pressures than females. Androgens have been strongly implicated in the hypertension of male SHR, because castration attenuates the hypertension. This study determined whether the androgen receptor plays a role in hypertension in male SHR and whether testosterone alone can cause the hypertension or whether conversion to dihydrotestosterone is necessary. Male SHR, aged 10 weeks, were given the androgen receptor antagonist flutamide (8 mg/kg SC; n=8) or the 5alpha-reductase inhibitor finasteride (30 mg x kg(-1) x d(-1) SC; n=11) daily for 5 to 6 weeks. Control rats (n=10) received vehicle (20% benzyl benzoate or ethanol in castor oil). After 5 to 6 weeks, blood pressure (mean arterial pressure) and glomerular filtration rate were measured. Long-term flutamide treatment caused a reduction in mean arterial pressure (control 178+/-5 mm Hg; flutamide 159+/-3 mm Hg; P<0.01), but finasteride had no effect (180+/-5 mm Hg). There were no differences in glomerular filtration rate among the groups. These data indicate that hypertension in male SHR is mediated via the androgen receptor and does not require conversion of testosterone to dihydrotestosterone.