Effects of oral L-carnitine supplementation on fat and protein metabolism have been studied in 20 low-birth-weight premature infants (mean weight at birth 1.519 g, range 1,200-1,880 g) fed with pooled pasteurized human milk. Throughout 7 consecutive days, started at various postnatal ages (range 10-33 days) infants were fed exclusively with milk containing 300 nmol/ml L-carnitine as added supplement. The amount of extra carnitine intake ranged from 42.6 to 72.0 mumol/kg/day. Until day 5 of supplementation there was a continuous increase in the daily urinary excretion of total carnitine, which levelled off thereafter, corresponding approximately to 50% of the extra L-carnitine intake, indicating that a part of the supplement was retained by the body. Total, free and esterified carnitine levels were significantly elevated in the plasma at the end of the study period. The increased levels of acylcarnitines in plasma and urine indicate that the carnitine supplement was taken up by tissues and entered the intermediary metabolism. Plasma triglyceride level was decreased, whereas 3-hydroxybutyrate level was increased at the end of supplementation, indicating an enhanced fat utilization. Plasma and urine analysis also revealed an altered nitrogen handling. There was a marked decrease in plasma urea level as well as a significant fall in the urea and total N excretion, with a trend of decrease in excretion of 3-methylhistidine, suggesting a reduced amino acid and protein catabolism during L-carnitine supplementation.