L-Carnitine has important roles in intermediary metabolism and patients with end-stage renal disease who are undergoing hemodialysis may develop a secondary L-carnitine deficiency. The extent of accumulation of the metabolites trimethylamine and trimethylamine-N-oxide when L-carnitine is administered orally has not been investigated previously in this population. Oral L-carnitine at a dose of 1 g daily was administered for twelve days to six patients with end-stage renal disease undergoing hemodialysis thrice weekly. Pre-dialysis plasma concentrations of L-carnitine (mean ± SD) increased significantly (P < 0.05) from day 1 (baseline; 32.4 ± 6.1 μM) to day 8 (66.1 ± 13.8 μM) remaining constant thereafter. Although plasma levels of trimethylamine remained unaltered, the pre-dialysis plasma concentrations of trimethylamine- N-oxide increased significantly (P < 0.05) from day 1 (289.1 ± 236.1 μM) to day 12 (529.0 ± 237.9 μM). The hemodialysis clearances for L-carnitine, trimethylamine and trimethylamine-N-oxide were 14.3 ± 8.2, 14.1 ± 10.6 and 12.4 ± 5.4 L/h, respectively, indicating their efficient removal by dialysis. Oral administration of L-carnitine at a dose of 1 g daily increases plasma concentrations of this substance to physiological levels in patients with end-stage renal disease who are undergoing hemodialysis. However, concerns about the possible deleterious consequences of such a dosage regimen still remain given that plasma concentrations of trimethylamine-N-oxide were continually rising and approximately doubled in a two-week period.