This report is based upon a study of 185 phonocardio-grams of individuals with normal hearts. For practical purposes, the 1st sound and the 2d sound should be divided into 3 phases: the slow vibrations which form the beginning and the end of the complex, and the main or middle portion of the complex which consists of large irregular vibrations that are caused chiefly by valvular elements. The duration of large vibrations as well as the duration of the entire sound complex should be measured. The avg. duraton of the 1st sound was 0.14 sec. at the apex and 0.14 sec. at the aortic area. The avg. duration of the middle phase of the 1st sound was 0.06 sec. at the apex and 0.063 sec. at the aortic area. The avg. duration of the 2d sound was 0.097 sec. at the apex and 0.104 sec. at the aortic area. The avg. duration of the middle of the second sound was 0.023 sec. at the apex and 0.039 sec. at the aortic area. The avg. duration of the 3d sound was 0.059 sec. at the apex and 0.042 sec. at the aortic area.