The effect of right ventricular pacing on left ventricular relaxation was studied in 13 patients (age 62 +/- 3 years), with the atrial sensing ventricular pacing mode (VDD). A control group of similar age (64 +/- 4 years) consisted of 11 patients with atrial pacing (AAI). The timing of events was determined in both groups at similar R-R intervals (921 +/- 77 ms vs 967 +/- 37 ms). The loading conditions as estimated by peak systolic wall stress (afterload) and end-diastolic left ventricular dimensions (preload) were approximately the same in both groups. The ratio of late to early filling velocities were similar in both groups. Dominant changes were: increased preejection period (142 +/- 13 ms vs 95 +/- 15 ms); and higher velocities of isovolumic relaxation flow (60 +/- 34 cm/s vs 25 +/- 4 cm/s) in patients with ventricular pacing. The isovolumic relaxation time was longer in patients with VDD pacing (127 +/- 14 ms vs 108 +/- 12 ms). Anterior systolic interventricular septal motion (paradoxal motion) was recorded in nine patients with VDD pacing and in none of the patients with AAI pacing. Isovolumic relaxation flow was detected during atrial pacing in five (45%) patients and in 13 (100%) patients during atrial sensing ventricular pacing, indicating asynchronous left ventricular relaxation. This data shows that VDD pacing compared to atrial pacing resulted in an altered activation pattern of the left ventricle, associated with delayed onset, asynchronous contraction with interventricular septal motion abnormalities and prolonged asynchronous left ventricular relaxation with abnormal motion manifested by the presence of isovolumic relaxation flow.