Abstract In order to find a simplified but reliable method for the determination of the effective renal plasma flow, several methods, employing p-aminohippurate (PAH) as well as p-acetylaminohippurate (PACAH), were investigated. It was found that the use of PAH is inappropriate for any method in which examination of the urine is omitted, since there is a considerable extrarenal loss of PAH. In contrast, the extrarenal loss of the acetylated compound of PAH (PACAH) was shown to be negligible. Using a constant infusion of PACAH the effective renal plasma flow could be determined from plasma PACAH concentrations and rate of infusion only, rendering the examination of urine unnecessary. The limitations of some other methods were evaluated.