In 10 patients without left heart valvular disease and having normal function of the left ventricle, the left ventricular apex cardiogram with its first derivative (dA/dt), left ventricular pressure with its first derivative (dP/dt), aortic pressure, electrocardiogram, and phonocardiogram were reocrded simultaneously during cardiac catheterization. The apex cardiographic tracings were obtained by means of a transducer with infinite time constant and very high resonant frequency and the LV and aortic pressures with catheter tip-manometers. The onset of the systolic rise of apex cardiographic and LV pressures were found to occur almost simultaneously with the upstroke of LV pressure, preceding that of the apex cardiogram by only 2 +/- 4 ms (mean +/- 1 SD). The summit of the systolic upstroke of the apex cardiogram (called E-point) occurred 37 +/- 9 ms after opening of the aortic valve and 41 +/- 9 ms after peak dP/dt. The peak of dA/dt preceded peak dP/dt by 10 +/- 4 ms. The protodiastolic nadir of the apex cardiogram (called-O-point) occurred slightly earlier (19 +/- 16 ms) than the nadir of the LV pressure curve, with considerable variation. In conclusion, this study using external and internal transducers with similar characteristics gives a new definition of the time relation between the externally recorded apex cardiogram and the haemodynamic events within the left heart in human subjects with normal left ventricular function.