The previously recognized variations in the timing and intensity of Mi, which may be associated with mitral valve pathology, myocardial decompensation, and with disturbances of conduction and rhythm, are briefly discussed and explained.Though it is generally accepted that the two components of the second heart sound depend on aortic and pulmonary valve closure (Leatham, 1954; Delman, i967), the number of components of the first sound and their mechanism of production remain disputed and ill understood (Dock, I965 Delman, I967). It has been shown by phonocardiography (Rappaport and Sprague, I942; Counihan et al., I95I; Dayem and Raftery, I966; Rushmer, 1970) that the normal first heart sound has at least 4 vibrations or groups of vibrations (Fig. i). These comprise initial low frequency vibrations, two higher pitched 'major vibrations', and then low frequency 'after vibrations.' Though the early low frequency vibrations are clinically audible in some instances (Kincaid-Smith and Barlow, I959a), it is the two major components only which are heard and assessed by most clinicians. The 'after vibrations' are probably never audible.