We studied plasma, dialysate, and muscle carnitine levels in patients with stable chronic renal failure on hemodialysis, and intermittent peritoneal, or continuous ambulatory peritoneal dialysis (CAPD). In patients on hemodialysis, plasma carnitine levels fell from 46.2 +/- 4.5 mumol/l (mean +/- SEM) to 18.8 +/- 2.7 mumol/l immediately after the procedure (p less than 0.001). Depletion of muscle carnitine was found after hemodialysis (1,518 +/- 273 nmol/g wet weight of tissue) compared to normal levels of 5,230.5 +/- 142.7 nmol/g tissue (p less than 0.01). However, the plasma and muscle carnitine levels remained in the normal range in patients on intermittent peritoneal dialysis and CAPD. We postulate that the rapid decline in plasma levels of carnitine caused by hemodialysis initiates unilateral transport of the compound from muscle to the plasma, thus depleting the skeletal muscle stores of carnitine.