To examine the Brody effect in humans, we studied 15 patients by means of coronary sinus pacing. We measured left ventricular (LV) volumes from the cardiac output (measured by the thermodilution technique) and LV ejection fraction (measured by radionuclide ventriculography). Pulmonary blood volume was determined by means of cardiac output and mean pulmonary transit time. In six patients, pacing was performed at two different rates, resulting in 21 pacing measurements. The heart rate increased with pacing from 73 ± 11 to 119 ± 19 bpm (mean ± standard deviation, p < 0.001). The end-diastolic volume (EDV) and the end-systolic volume (ESV) decreased with pacing (p < 0.001 each). The R wave amplitude decreased with pacing (1.44 ± 0.63 mV control vs 1.32 ± 0.58 mV with pacing; p < 0.01). R wave amplitude decreased in 19 of the 21 pacing studies (90%); EDV and ESV decreased in all 21 pacing studies, and pulmonary blood volume decreased in 14 of the 15 pacing studies (93%) performed in 11 patients. There was a significant correlation between the percentage of change in R wave amplitude with the percentage of change in EDV (r = 0.54, p < 0.01) and with the percentage of change in ESV (r = 0.54, p < 0.01). These results, therefore, validate Brody's hypothesis and indicate that changes in LV volumes affect the R wave amplitude. © 1984.