Repeated measurements of cardiac output in the same 11 normal individuals were done with $^{131}I-Hippuran$ and RIHSA. Following results were obtained; 1) The cardiac output measured with RIHSA was $6750{pm}866ml/min$. 2) The cardiac output measured with $^{131}I-Hippuran$ was $5940{pm}764ml/min$. 3) The relative value of cardiac output measured with $^{131}I-Hippuran$ to that measured with RIHSA was 88 percent, and a statistical significance was found present in this difference.