Background The purpose of this study was to test the hypothesis that vasodilator responses of porcine coronary resistance arteries are increased by exercise training.Methods and Results Yucatan miniature swine were randomly divided into groups of exercise-trained (ET) and sedentary (SED) control pigs. ET pigs were placed on a progressive treadmill training program lasting 16 to 20 weeks, and SED pigs remained inactive during the same time period. Coronary resistance arteries 64 to 157 microns in diameter were isolated for in vitro evaluation of relaxation responses to the endothelium-independent dilators sodium nitroprusside (1 x 10-10to 1 x 10-4mol/L) and adenosine (1 x 10-10to 1 x 10-5mol/L) and to bradykinin (1 x 10-13to 3 x 10-7mol/L), an endothelium-dependent agent. Relaxation responses to adenosine and sodium nitroprusside were not altered by exercise training. Endothelium-dependent relaxation to bradykinin was enhanced in coronary resistance arteries from ET pigs (IC50: ET, 0.07+-0.02 nmol/L; SED, 1.59+-0.09 nmol/L). To determine whether prostanoids and/or the nitric oxide synthase pathway were involved in the ET-induced changes in bradykinin-induced vasodilation, responses to bradykinin were examined in coronary resistance arteries from both ET and SED pigs in the presence of indomethacin and in the presence of nitro-monomethyl l-arginine (L-NMMA). Both indomethacin and L-NMMA produced significant inhibition of the bradykinin-induced relaxation in vessels from both groups. Despite decreased bradykinin-induced relaxation after indomethacin, bradykinin-induced vasodilation was still enhanced in vessels from the ET group. L-NMMA caused greater inhibition of the bradykinin-induced relaxation in coronary resistance arteries from ET pigs relative to arteries from SED pigs and eliminated the training-induced enhancement of the bradykinin responses.Conclusions These results suggest that exercise training enhances bradykinin-induced vasodilation through increased endothelium-derived relaxing factor/nitric oxide production by the l-arginine/nitric oxide synthase pathway. (Circulation. 1994;89:2308-2314.)