[Methyl-3H]epsilon-N-trimethyl-L-lysine was administered to patients with primary systemic carnitine deficiency and to controls. In both groups, labeled carnitine appeared in blood and urine within 2 hours, and the specific radioactivity of urinary carnitine peaked between 2 and 6 hours. The specific radioactivity of serum carnitine peaked at 6 hours in the controls and in one patient, but in the other patient it rose sharply in the first 2 hours, fell slightly, and then gradually increased to a maximum at 48 hours. The 48-hour excretion of [methyl-3H]-L-carnitine was 4.4 to 6.0 muCi for the controls and 24.2 and 5.6 muCi for patients A and B, respectively. Eight other radioactive metabolites were found in urines of all subjects. Each metabolite was present in comparable amounts. Primary systemic carnitine deficiency in these patients did not result from defective biosynthesis or abnormal degradation of carnitine.