Eighteen breast-fed infants with a mean post-natal age of 26 days (range 16-41 days) were studied. Following 1 control day, the infants were fed for 7 consecutive days with pooled human milk supplemented with 300 nmol L-carnitine/ml milk. Both plasma fractions of acid-soluble carnitines increased as a consequence of carnitine application. The level of beta-hydroxybutyrate also increased significantly. Of the circulating free amino acids, the levels of alanine (p less than 0.025) and glutamine (p less than 0.01) were found to be lower, with a decreased urea level (p less than 0.005) by the last day of carnitine administration, compared with the control day. The urinary output of total nitrogen also decreased. There were no statistically significant changes in the level of free fatty acids and glucose. On the control day, the renal clearance rate of esterified carnitines significantly exceeded that of free fraction, thus the relative renal reabsorption calculated on the base of creatinine excretion rates was higher for free (mean 98.1%) than for acylated (mean 90.6%) carnitine. In response to enhanced carnitine intake, the clearance rates for each fractions of carnitines significantly exceeded the presupplementary values. The increased clearance rates was more pronounced for free (mean 13.2-fold) than for esterified (mean 8.08-fold) carnitines. Despite the increased clearance rates, considerable relative reabsorption was seen for free carnitines (mean 70.0%) as well as for acylcarnitines (mean 65.3%).