Sunagawa and coworkers have proposed an ejecting, single beat, cosine curve matching method for the estimation of the maximum potential left ventricular pressure during an isovolumic contraction. We tested the hypothesis that this same information could be obtained from maximum dp/dt and the length of the ventricular contraction curve using a simple formula. The method was tested using Sunagawa's reported data as well as on data obtained from five dogs over a wide range of afterloads, and from three rabbits, four pigs, and four sheep. A high correlation coefficient between the two methods (.85 to .97), based on a linear model, was obtained in all the experiments. The formula method gave slightly higher values than the cosine curve matching procedure, was much simpler, and involved few of the curve matching assumptions. The maximum left ventricular pressure was also highly correlated with stroke work. These results imply that the maximum potential left ventricular pressure or energy is represented in, and can be predicted from, the ventricular pressure during normal ejection at a given end diastolic volume. When combined with measurements of end systolic pressure and stroke volume, reliable beat-to-beat estimates of the slope of the end systolic pressure-volume relationship (end systolic elastance) might be obtainable from single, left ventricular beats.