The metabolism, excretion and disposition of melamine were determined after administration of a single oral dose of 0.025 mCi (0.38 mg) [14C]melamine to adult male Fischer 344 rats. Within the first 24 hr, 90% of the administered dose was excreted in the urine. Negligible radioactivity appeared in breath and faeces. There was little difference in blood, liver or plasma concentrations of 14C, suggesting that melamine distributes in body water. The only organs showing radioactivity levels much higher than plasma were the kidney and bladder. The bladder level was by far the highest, a finding probably due either to back diffusion from urine or to contamination of bladder tissue with urine. Virtually no residual radioactivity was observed in tissues examined at 24 hr or later. The elimination-phase half-life calculated from plasma data, 2.7 hr, was in good agreement with the urinary-excretion half-life of 3.0 hr. The renal clearance of melamine was 2.5 ml/min. Radioactivity in plasma or urine co-chromatographed with that of the dosing solution, indicating that melamine is not metabolized in the male Fischer 344 rat.