Right ventricular ejection fraction and tricuspid regurgitation were estimated by radionuclide angiocardiography to quantitate systemic ventricular function in "simple" transposition of the great arteries. Mean right ventricular ejection fraction was 0.52 +/- 0.07 for 18 pre Mustard operation infants, 0.54 +/- 0.07 for 23 patients operated upon less than a 1 year before, and 0.57 +/- 0.08 for 14 patients operated upon more than 3 years earlier. Eight patients were evaluated before and following the Mustard operation; ejection fraction rose in three, fell in three, and remained constant in two. Mean right ventricular ejection fraction was not different between groups nor when compared to our "normal." The left ventricle:right ventricle stroke volume ratio of the postoperative patients was compatible with tricuspid regurgitation in four patients. This radionuclide study suggests that following surgery for transposition of the great arteries: mean right ventricular systolic ejection fraction remains at levels consistent with values usually found for the "normal" right ventricle; group right ventricular function does not deteriorate in the years following surgery; and tricuspid regurgitation may be detected in the early postoperative years.