To better understand biventricular mechanics, an algorithm was developed to simultaneously calculate right and left ventricular volumes from randomly placed subendocardial radiopaque markers. Mathematically, the ventricle is represented as a stack of circular discs. The radius R of each disc is calculated as the distance from the subendocardial radiopaque marker to a computer generated base-to-apex line, and the height H of each disc is determined by the projected distance between radiopaque markers along the base-to-apex line. Accordingly, the volume (V) is calculated as V = pi . sigma Hi . Ri2. The validity of this algorithm was tested on 10 canine left ventricular casts, on 10 human right ventricular casts, and in five experiments. For the left ventricle, the regression line between the casts (VT) and calculated (VC) volumes was VC = 0.55 VT + 6.6, with r = 0.95, standard error of estimate (Sy) = 1.9 ml, and the standard deviation of percent error = 12.6%. For the right ventricle, VC = 1.75 VT = 42.5, with r = 0.86, Sy = 16.2 ml, and the standard deviation of percent error = 24.8%. In five animal experiments, radiopaque markers were implanted into the endocardium of the left and right ventricles and comparisons were made between angiographic- and marker-determined ventricular volumes. For the five experiments, the mean correlation coefficient, relating the marker volumes to the angiographic volumes, were 0.92 +/- 0.01 for the left ventricle and 0.89 +/- 0.02 for the right ventricle. The results, which are similar to other volume-determination methods, indicate that this method can be applied to determine right and left ventricular volume. Once implanted, fluoroscopy of these markers provides a noninvasive means of calculating ventricular volume.