For the clearance investigations of children who underwent renal transplant operations methods without indwelling catheters in the urinary bladder are necessary. The determination of GFR and ERPF takes place by means of single injection of radioactively labelled clearance substances. The maximum tubular secretory function may be established by means of the Tm PAH. Up to now for this purpose the quantitative determination of PAH in the urine was necessary. By means of an on principle other method by determination of the 125J-hippuran clearance under infusion of PAH the Tm-PAH may be indirectly calculated without investigation of the urine. In nine children with renal diseases the parallel determination was performed with classical and new method (r = 0.89, p less than 0.01), in order to prove the good correspondence of the results of the two methods. The Tm PAH in seven children who underwent renal transplant operations shows at first an at ERPF relatively decreased, after several years a relatively increased Tm PAH.