SummaryVarious methods were compared of studying the vascular phase of the radiohippuran renogram in man. Radioalbumin which, during the short time interval studied, does not leave the circulation, was used to evaluate the vascular capacity of the kidney region: Approximately 2.48 times more radioactivity was observed during the vascular phase of the radiohippuran renogram than during the time interval when radioalbumin was contained in the vascular bed of the kidney region. This ratio was arrived at by investigating nephrectomized subjects, thereby eliminating the effects of radioactivity in extrarenal vessels of the kidney region. The higher radioactivity over the kidney region during the vascular phase, shortly after a single radiohippuran dose has been given, could be explained by a rapid secretory process but is more likely due to the presence of a renal mechanism that concentrates hippuran. This “hippuran trapping” would occur between the “vascular” and the secretory phases. In renal function studies with radiohippuran the additional use of radioalbumin to evaluate the vascular capacity of the renal region appears to be of advantage.