The right ventricular ejection fraction (RVEF) was determined on the right anterior oblique view in 9 patients during the first pass of a bolus of technetium-99m employing a gamma camera with high count-rate capability. The RVEF was calculated by using: (i) a fixed end-diastolic region of interest (ROI); and (ii) an end-diastolic and end-systolic ROI. Because of the movement of the tricuspid plane the first of these methods often gave low values, and agreement between the first two peaks was not as good as that when the second method was used. The mean for the second method was in agreement with that in a previous study using a gated first pass technique and two ROIs but was somewhat higher than those reported by workers using either one ROI or the anterior view.