OBJECTIVE The ability of the thyroid hormone to increase cardiac output and to lower systemic vascular resistance may provide a novel treatment for cardiovascular diseases. Therefore, understanding the mechanisms of thyroid hormone action on the heart and peripheral vasculature could be of clinical importance. We previously found that thyroid hormone modulates the alpha1-adrenergic effect on vascular reactivity of rat aortas. In the present study we further investigated possible mechanisms of this response. METHODS Hyperthyroidism was induced on Wistar-Kyoto male rats with L-Thyroxine, (THYR) treatment for two weeks, N.=18 while untreated rats used as controls (NORM), N.=16. The thoracic aorta was dissected and cut into rings that were suspended in an isolated organ bath with Krebs-Henseleit buffer. Maximal tension, Tmax, in g was measured in response to Potassium Chloride (KCl) and Phenylephrine (PE) in rings in the presence of Ritodrine, a beta-2 agonist (NORM-RITO, N:=8, THYR-RITO, N.=9), or in the absence of Ritodrine (THYR, N.=9, NORM, N.=8). RESULTS With KCL, Tmax was not different between the THYR, NORM, NORM-RITO, and THYR-RITO groups. With PE, there was a difference in Tmax between NORM-RITO and NORM, 0.66 (0.056) g vs 1.00 (0.066) g, P<0.05 and THYR and NORM, 0.75 (0.055) g vs 1.00 (0.066) g, P<0.05. No significant difference was observed between THYR-RITO AND THYR. Furthermore, Relax % was not significantly different between the NORM and the THYR, NORM-RITO, and THYR-RITO groups, 64.5%(3.7) vs 67.3%(6.7), 73.5% (4.3) and 81.8 %(4.7), P>0.05. CONCLUSIONS PE induced vasoconstriction in isolated rat aortic rings was reduced after both ritodrine and thyroxine treatment. However, co-administration of thyroid hormone and ritodrine did not result in a synergistic reduction of PE induced vasoconstriction. Thus, thyroxine may modulate the alpha1-adrenergic vascular responsiveness by enhancing beta2-adrenergic stimulation.