Echocardiographic studies were performed in order to examine the effect of preceding R-R interval (PI) and pre-preceding R-R interval (PPI) on the changes in end-diastolic and end-systolic dimensions (Dd and Ds) in 12 patients with atrial fibrillation. As PI prolonged, Dd increased and Ds decreased, however, these were constant at the PI of more than 0.80 up to 1.20 sec. At the PI of less than 0.80 sec, Dd and Ds showed a wider range of distribution. When PPI remained constant, Dd increased and Ds decreased with the prolongation of PI. When PI was kept constant, Dd increased and Ds decreased as PPI became shorter. The greater the degree of PPI shortening and PI prolongation, the larger the increase of Dd and decrease Ds. Thus, the change of contractile state of the left ventricle in atrial fibrillation was affected not only by the preload of the individual PI, but also by that of the PPI. This is thought to be the same mechanism as in post-extrasystolic potentiation.