To evalulate the relation between ventricular endocardial potentials (QRS amplitude) and ventricular dimensions, left and right ventricular endocardial potentials were recorded with hook electrodes in anaesthetised open-chest dogs during transfusion and withdrawal of blood. Left ventricular end-diastolic diameter was measured by ultrasonic crystals, and end-diastolic volume was determined by thermodilution. In each dog, left ventricular endocardial potentials, whether recorded from anterior or posterolateral walls, decreased linearly as left ventricular end-diastolic diameter or volume increased, and vice versa. With an average increase in left ventricular end-diastolic diameter from 40.1 +/- 1.7 to 44.6 +/- 1.8 mm, left ventricular endocardial potentials decreased from 32.8 +/- 2.5 to 23.5 +/- 2.3 mV (P less than 0.001); and for an increase in left ventricular end-diastolic volume from 1.36 "/- 1.25 to 3.43 +/- 0.58 ml/kg left ventricular endocardial potentials decreased from 36.2 +/- 6.6 to 14.9 +/- 4.3 mV (P less than 0.001). Changes in right ventricular endocardial potentials paralleled the changes in left ventricular endocardial potentials. These findings indicate that acute changes in ventricular dimensions influence endocardial potentials considerably, and suggest a potential clinical application for detecting acute changes in ventricular volume.