The kidney forms arginine (Arg) by using citrulline (Cit) as precursor, and is the main source of Arg for systemic protein synthesis. Even if the filtered and reabsorbed load (in rats) is sufficient for normal Arg synthesis, the following questions remain. (a) Can Cit be taken up across the contraluminal membrane of the tubule cells also? If so, (b) by what kind of mechanism? And (c) is this Cit, entering the cell from the peritubular side, metabolized to Arg and ornithine (Orn)? Although these questions are raised mainly in connection with mammals, we used the amphibian kidney, which is especially suitable because of its double blood supply, for an initial approach to the problem. After the toad was decapitated, the portal vein, the caval vein and the ureters were catheterized, and the kidneys were perfused through the portal vein (Ringer solution+ L-or D-Cit+inulin+p-aminohip-purate+L-aspartate). Exclusive peritubular perfusion was assured by showing that inulin perfused into the portal vein did not appear in the urine. During perfusion of the portal vein with L-Cit in a physiological concentration (65 ~tmol/1), an initial peritubular net uptake of L-Cit of 170+27 (n=10)nmol'h-i-g kidney -~ (wet weight) was observed, Whereas the value for D-Cit (65 Ixmol/1) was only 18 + 7 (n = 6) nmol" h-1. g-1. After perfusion for 50min, the uptake of L-Cit reached a steady state with an uptake rate of 108 + 5 nmol. h-i. g-1. Adding L-phenylalanine (L-Phe; 20 mmol/1) to the solution or substituting mannitol for NaC1 in the perfusate, decreased this L-Cit uptake to values similar to those for D-Cit. During perfusion with 65 ~tmol/1 L-Cit, the Arg delivery into the venous blood was 40+4.8nmol.h-~.g -l, corresponding to 36% of the peritubular Cit uptake in the steady state, and the Orn delivery was 49.6 + 3.2 nmol. h-2. g-1, corresponding to 46% of peritubular Cit uptake in the steady state. The venous Arg delivery increased to 51 nmol.h-l.g -1 while * Parts of this work have been reported at the 19th congress of the Deutsche Gesellschaft ft~r Nephrologie in GOttingen (FRG), 1988 [12] Offprint requests to: S. Silbernagl the kidney was perfused with 1 mmol/l L-Cit. At higher L-Cit concentrations no further increase of venous Arg delivery could be observed. During perfusion with D-Cit (65 Ixmol/1) or when adding L-Phe (20 mmol/1) no Arg or Orn could be detected in the venous outflow. In conclusion, the amphibian kidney perfused in situ is a suitable model for studying peritubular amino acid uptake and metabolism in the kidney. In the toad kidney, a peritubular uptake mechanism for L-Cit exists, which is stereospecific, saturable, NaCl-dependent and can be inhibited by L-Phe. In the tubule cells, L-Cit is transformed into Arg. Part of the Arg is metabolized further to Orn and urea, and Arg and Orn are released into the venous outflow.