Carnitine plasma concentrations were determined by an enzymatic radioisotopic method in 353 metabolically healthy children and in 41 adults. There was a positive correlation between total and free carnitine plasma concentrations and the age of the children. Both free and acylcarnitine concentrations were elevated on the 1st day of life, reflecting an increased rate of fatty acid oxidation. Carnitine plasma concentrations decreased after the 1st day and subsequently increased during the 1st year. From the 2nd year of life until adulthood, no further change was noted. Up to 17 years of age no differences were seen between male and female individuals. However, adult males had higher carnitine concentrations in plasma than adult females. Total carnitine concentrations were higher in 10- to 17-year-old females and lower in 10- to 17-year-old males compared with adults of the same sex, indicating a possible role for sex hormones in the regulation of carnitine plasma concentrations.