In order to study the intracardiac murmurs in coronary venous system, right heart catheterization was carried out on 35 patients with or without mitral regurgitation. The double-lumen phonocatheter of A. E. L. was used for 33 cases but for 2 the microtip phonocatheter of Millar was employed. As a rule, simultaneous recording of intracardiac and external phonocardiograms was made with the pressure tracing in the majority of cases. The examined subjects were divided into 3 groups; 9 cases with mitral regurgitation confirmed by left ventriculography (Group I), 20 without mitral regurgitation (Group II), and 6 with functional systolic murmur (Group III). In Group I, a loud systolic murmur was recorded in 7 cases (MR 5, MR + AR 1, ECD 1), but in none of IHSS in the coronary sinus or great coronary vein. In Group II, no systolic bruit was noted in VSD and other lesions except ASR in the coronary venous system. The same was true in patients with functional systolic murmur (Group III). Intracardiac phonocardiography is thought to be useful to record mitral regurgitant murmurs in the coronary venous system, since the latter is in the close proximity to the mitral posterior commissure or mitral annulus.