The renal tubular handling of free amino acids was studied 5-6 weeks after successful renal transplantation (tx) in 20 children treated with CsA and in 10 children treated with azathioprine (Aza). The results were compared with those of 34 control children. The amino-acid clearance studies were performed in combination with short-term inulin clearance. The CsA group revealed a mean inulin clearance of 49 +/- 16.8 ml/min/1.73 m2, the Aza group of 76.9 +/- 18.2, and the controls of 114 +/- 15.6. The plasma amino-acid concentrations were not different between CsA- and Aza-treated groups; however, most of the essential amino acids were lower in transplanted children than in controls. The decrease was correlated with the GFR. The amino-acid-clearance rates were statistically not different between both transplanted groups, but lower values than in controls were found for alanine, glycine, histidine, lysine, and phenylalanine, and significantly higher values for methionine. The fractional clearance rates of most amino acids were significantly elevated in transplanted children compared to controls. In CsA-treated patients, the fractional clearance rates of arginine, glycine, and serine were higher than in Aza-treated patients. No influence of CsA blood levels or rejection episodes on the amino-acid handling were detectable. We conclude that CsA has no specific influence on the renal handling of amino acids. Most disturbances observed depend on the graft function or may be caused by injuries to the graft following the tx procedure.