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Ventricular fibrillation was induced in 56 human subjects on cardiopulmonary bypass by applying 50 Hz sinusoidal currents to the heart using three electrode systems: epicardial disc electrodes, intramyocardial needles and endocardial electrodes. By plotting the results as a cumulative distribution curve, it was shown that there is a linear relationship between the percentage of people fibrillated at a given current level and the logarithm of the current applied. The correlation coefficient was higher than 0·94 in each series. An extrapolation of the line of best fit allowed an estimate of the minimum current required to produce ventricular fibrillation in the population examined. Using epicardial disc electrodes, the predicted minimum fibrillating current was 735 microamperes RMS, with 99% confidence limits of less than 2 microamperes. Using intramyocardial needle electrodes, the predicted minimum fibrillating current was 199 microamperes with 99% confidence limits of 1 microampere. Using endocardial electrodes, the predicted minimum fibrillating current was 67 microamperes with 99% confidence limits of less than 2 microamperes. In one subject with grossly abnormal electrical activity of the heart, ventricular fibrillation was probably induced with 15 microamperes. These results and their significance are discussed.