Plasma total and free carnitine concentrations were measured in 14 patients during a 4-hour hemodialysis session at 40-min intervals. Although postdialysis carnitine levels were lower than predialysis levels, they did not drop progressively and rebound occurred early or late. While no relationship was seen between pre- and postdialysis carnitine values, an inverse correlation was found when the predialysis value was compared with an end-dialysis to predialysis ratio. Our data are not consistent with a first-order elimination kinetics of the molecule and suggest that during dialysis carnitine may be actively released from tissue stores to plasma pool. The finding of an inverse correlation between the fractional change (post-pre ratio) of plasma carnitine and its predialysis level suggests that plasma carnitine supply from cells is more active when plasma carnitine levels are lower.