A plethysmograph for measuring ventricular stroke volume (SV) from the transthoracic admittance variation was developed. A correlation study between the SV values determined by this method (SVy) and those measured by Kubicek's transthoracic impedance method (SVz) was carried out on 24 healthy subjects; the correlation coefficient was r = 0.99 and the regression line of SVY on SVz was y = 1.03x - 0.47. Based on the results obtained through the study, it was concluded that the admittance method was more advantageous than the impedance method for measuring SV.