Acetyl-L-carnitine 1.5 g and 3.0 g was administered as three divided doses on each of two occasions to 24 people with varying renal failure (creatinine clearance 127-8 ml.min-1). Plasma and urinary concentrations of total-L-carnitine, free (non-esterified) carnitine, short-chain esters and acetyl-L-carnitine were measured. The baseline (pre-study) concentrations of all four substances were related to renal function. Patients whose creatinine clearance was below about 30-40 ml.min-1 were had the highest concentrations. Renal elimination of all four substances was related to dose and to renal function. There was evidence for dose-related elimination, with greater elimination of the larger dose.