Cervical vagal stimulation in rabbits frequently causes systolic murmur with bigeminy due to premature ventricular contractions. The bigeminy disappears in a few minutes, but systolic murmur persists for a few days. Peculiar lesions of the mitral valves, mitral annulus and papillary muscles, and increased left atrial weight, develop in a week. Color Doppler echocardiography was used to examine whether the systolic murmur was due to mitral regurgitation. Electrocardiography was performed in anesthetized rabbits restrained in the supine position. Doppler echocardiography and phonocardiography showed systolic murmur at 6 h, three days, and at one, two, three and four weeks after vagal stimulation. At 6 h after stimulation, phonocardiography showed systolic click and late systolic murmur; Doppler echocardiography showed marked mitral regurgitation. The systolic murmur and mitral regurgitation were attenuated and the papillary muscle was swollen three days after vagal stimulation. Mitral regurgitation disappeared within one week of vagal stimulation, and papillary muscle swelling improved after three weeks. In conclusion, Doppler echocardiography confirmed that the systolic murmur observed after vagal stimulation in rabbits was due to mitral regurgitation.