Several recent studies have provided evidence that gonadal steroid hormones can exert acute (nongenomic) effects on both neural and vascular tissues. This study examines the acute effects of testosterone (T) on vascular reactivity of the rat thoracic aorta. Aortic rings from male Sprague-Dawley (SD) rats with (+ENDO) and without (-ENDO) endothelium were prepared for isometric tension recording. In (+ENDO) male aortae precontracted with phenylephrine (PE), T produced dose-dependent relaxation from 25 microM (30.3 +/- 7.1%) to 300 microM (99.4 +/- 0.4%), whereas T vehicle (< or = 0.5% ethanol) had no effect. Pretreatment of (+ENDO) aortae with T (50 microM; 10 min) attenuated subsequent contractile responses to PE. Both maximal contraction and sensitivity to PE were reduced by T. Pretreatment of (+ENDO) aortae with both T and N omega-nitro-L-arginine methyl ester (250 microM) reversed in part the attenuating effects of T alone; however, both maximal response and sensitivity to PE were still reduced compared to control rings (without T or N omega-nitro-L-arginine methyl ester). Pretreatment of (-ENDO) aortae with T reduced sensitivity to PE, but had no effect on maximal contraction. T pretreatment (50 microM; 10 min) of both (+ENDO) female SD aortae and (+ENDO) male testicular-feminized rat aortae reduced maximal contraction and sensitivity to PE in both groups to a similar extent as in (+ENDO) male SD aortae. These data suggest that T has a direct vasodilating effect on the rat aorta, which involves endothelium-dependent (enhanced NO release) and -independent mechanisms and is gender- and intracellular androgen receptor-independent.