The time relationship between left apexcardiogram and left ventricular pressure was studied in 56 patients with heart disease. The apexcardiogram was recorded by means of a pressure transducer in 20 patients (group I). In 36 patients a displacement transducer was used (group II). The beginnings of the systolic upstroke of left ventricular pressure and left apexcardiogram were nearly simultaneous in both groups. The protodiastolic nadir of the apex tracing, however, preceeded the corresponding point on the pressure tracings by 23.8 msec in group I and 7.8 msec in group II (mean values). The peak of the first derivative of the apexcardiogram in group II preceeded the peak of the first derivative of the intraventricular pressure by a mean of 13.4 msec. In group I the time difference was small and statistically insignificant. It is concluded that apexcardiography allows an accurate determination of the onset and time course of ventricular contraction but is less precise for the determination of the protodiastolic nadir.