[en] Gated radionuclide imaging with /sup 99m/Tc-labeled red blood cells was employed to assess changes in right atrial (RA) volume in 17 subjects. Studies were repeated within two hours in five of 12 normal subjects and at two and ten days after infarction in five patients. Time-activity curves were generated for the RA and right ventricle (RV) from variable regions of interest defined by automatic edge detection. The RA time-activity curve exhibited four phases: filling, early emptying, equilibration, and late emptying. Thus, relative changes in right atrial volume can be assessed reproducibly by a radionuclide technique, which is relatively independent of geometric assumptions. This technique offers great promise as a method to assess atrial function in health and disease