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The distribution of arginine synthase and arginase activities along the successive nephron segments of Meriones kidney was measured in vitro with single tubule enzymatic microtechniques making use of either L-[ureido-C-14] Citrulline (0.108 mM) or L-[guanidino-C-14] arginine (0.2 mM) as the respective substrates. Arginase activity (fmol urea formed per min per mm of tubule) was very low (5-25 fmol . min(-1). mm(-1)) in most nephron segments including the early portions of proximal convoluted tubules (early PCT). It increased progressively after 3 mm of the PCT to reach a value of 200 fmol . min(-1). mm(-1) in the cortical portion of the straight proximal tubule (CPST), with a further increase, along the pars recta, of up to 250 fmol . min(-1). mm(-1) in the outer medullary portion (OSPST). In addition, arginase activity in OSPST and the adjacent descending thin limb (DTL) was higher in juxtamedullary nephrons (JN) than in the corresponding portions of superficial nephrons (SN). Arginine synthase activity (fmol arginine formed per mm of tubule per min) was present in proximal tubules exclusively, with a gradient decreasing along the PCT (about 600 fmol . min(-1). mm(-1) in the 1st mm, 65 fmol . min(-1). mm(-1) in CPST and 30 fmol . min(1)(-) . mm(-1) in OSPST). It has been checked that CPST and OSPST (where the two enzyme systems are present) are able to convert citrulline directly into urea with a yield of 65%. It is suggested that: (1) in early PCT cells, arginine synthase activity permits the conversion of the reabsorbed citrulline into arginine (which then diffuses towards blood vessels); and (2) in pars recta cells, arginase activity results in a net entry of arginine across the basolateral membranes and in a net exit of the formed urea into the tubular fluid, if the permeability