Haemodynamic studies suggest that the rapid deceleration of left ventricular inflow at the end of early diastole may play an important role in the genesis of the third heart sound (S3). To confirm this hypothesis noninvasively, pulsed Doppler of transmitral flow was used. Mitral flow velocity wave was recorded in 20 post-infarction patients with audible S3 (Group 1), in 20 young healthy individuals with physiologic S3 (Group II), in 20 postinfarction patients without S3 (Group III) and in 20 normal adults (Group IV). Peak flow velocity in early diastole (Ev), peak flow velocity during atrial systole (Av), the Ev/Av ratio, the deceleration of early diastolic flow (EF slope), the ratio of the time velocity integral of early diastole to the total time velocity integral (TVle/TVlt) and the isovolumic relaxation time (IVRT) were measured from Doppler recordings. The time relation between S3, the mitral valve motion on M-mode tracing, and the mitral flow velocity wave were analysed comparing the intervals from the second sound to Ev (A2-Ev), to the E point of mitral valve motion (A2-Em) and to the S3 (A2-S3). In groups I and II Ev/Av ratio was higher (respectively 4.4 +/- 2.2 and 2.8 +/- 1.1) than in group III (0.8 +/- 0.4) and IV (1.3 +/- 0.3). Similar results were found for the TVle/TVlt ratio. In both groups with S3, EF slope was significantly steeper (respectively 9 +/- 1.8 and 7.5 +/- 1.1 m s) than in normal adults (4.4 +/- 1.1 m s) and patients without S3 (3.6 +/- 1.1 m s).(ABSTRACT TRUNCATED AT 250 WORDS)