Carnitine was determined radioenzymatically in the plasma of 415 hospital employees involved in a screening programme for prevention of major cardiovascular risks. A reference population (N = 340) was extracted after excluding subjects with hypertension, diabetes mellitus or treatment for hypercholesterolaemia. This population showed a Gaussian distribution for total and free carnitine concentrations both in females and males but not for acyl carnitine or the acyl/free ratio. Females had lower total and free carnitine concentrations but a higher ratio of acyl/free carnitine than males. These differences were not detectable in older subjects (35 years for the acyl/free ratio, 45 years for total and free carnitine concentrations). Females with a body mass index > 28 had a lower acyl/free ratio than their respective controls. The differences in carnitine concentrations indicate that sex and age should be matched in patients or experimental groups and controls in studies involving carnitine plasma concentrations.