To assess the mixing characteristics of the right ventricle and pulmonary artery, radioiodinated (131)I serum albumin and indocyanine green dye were injected simultaneously in 16 subjects. One indicator was injected into the atrium and the other into the ventricle, or both were injected at different sites in the ventricle. Washout curves were obtained by rapid catheter sampling alternately just above or just below the pulmonic valve. The washout of radioisotope was also recorded with a precordial scintillation detection probe. Indicator washout from the ventricular inflow tract was rapid, while washout from the region of the ventricular apex was quite slow. Protosystolic dips in indicator concentration, noted in curves drawn below the pulmonic valve, suggest that the ventricle emptied sequentially. Flow values computed from curves sampled below the valve, when compared with reference values, suggest that a significant volume of atrial blood passed through the ventricle without mixing, or mixing to only a small extent, with the residual volume of the chamber. Peak concentration of indicator was higher below the pulmonic valve than above. This finding, plus the close agreement between flow values computed from curves sampled above the valve and the reference values, indicates that further mixing occurred above the valve. Ventricular volumes computed from washout downslopes are systematically falsely high. This overestimate appeared to be larger in normal subjects than in patients with low stroke volumes. Progressive mixing of blood leaving the atrium occurs during its passage through the ventricle, pulmonic valve, and pulmonary artery and permits accurate estimation of flow.