Control ranges of free, total and esterified carnitine in serum and spot urine samples of a healthy population of children and adults were established by radio-enzymatic assay. Children were divided into three groups according to age: six months to two years (n = 15), three to five years (n = 19), and six to ten years (n = 16). The adult group ranged in age from 20 to 35 years (n = 18). Serum free and total carnitine displayed a significant increase up to ten years of age, while acylcarnitines and the acylcarnitine/free carnitine ratio decreased until five years of age. Urinary free carnitine increased significantly up to five years of age. The overall increase in urinary total carnitine was insignificant. Urinary acylcarnitines and the acylcarnitine/free carnitine ratio decreased significantly in the three- to five-years age-group, increased significantly in the six- to ten-years age-group and remained constant afterwards. Renal handling of carnitine was assessed by evaluation of the renal reabsorption rate for free carnitine (RRFC) and the acylcarnitine/free carnitine clearance ratio (RAFCC), both of which were found to increase significantly up to five years of age. These data suggest that a balance in carnitine metabolism and excretion seems to be reached only after five years of age. The establishment of reference ranges in different population groups is critical for the investigation of children suspected to suffer from inherited metabolic disorders.