(1) The endogenous hippuric acid can be fluorometrically determined down to a minimum of 0.6 micrograms/ml in 10 microliters of rat serum. This enables the determination of the effective renal plasma flow (ERPF) by measurement of the renal clearance of the endogenous hippuric acid in the conscious rat. (2) Endogenous hippuric acid clearance (128 ml/min . m2 or 2.48 ml/min/100 g body weight) corresponds in its extent with the clearance after a single injection of unlabelled hippuric acid and is higher (20%) than the clearance after a single injection of 131I-hippuran. (3) Exogenous hippuric acid has a short biological half-time of 12-20 min and a distribution space of about 50%; therefore, alteration of its intermediary supply can disturb the estimate of the renal clearance of endogenous hippuric acid. (4) The daily rhythm of the renal excretion of endogenous hippuric acid shows a constant low excretion between 0800 and 1200 hours; this period is the best suited for clearance measurements.