A total of 29 preterm infants maintained on mixed enteral nutrition (50% pooled human milk, 50% formula daily) were studied over a 15 days period. 16 of them received L-carnitine supplemented formula during the first seven consecutive days (600 nmol/ml, as added supplement), 13 infants served as controls. In response to enhanced dietary intake, the plasma levels and urinary excretion rates of carnitines were increased by the 7th day of study. The plasma carnitines then returned to the initial values, whilst the urinary excretion remained elevated at the 14th day of study. The elevated daily urinary excretion of carnitines was accompanied by increased clearance and decreased relative reabsorption rates in the supplemented group. In the control group the plasma carnitine levels remained unchanged throughout the observations, while the daily excretion of free carnitine decreased by the end of the study. In the supplemented group statistically significant decrease was found in the daily excreted ammonia and urea with a decrease of plasma alanine and glutamine levels by the 7th day of study. The plasma levels of beta-hydroxybutyrate, glucose and creatinine remained unchanged in both groups.