Plasma levels of total, free and acylcarnitine, as well as oxygen consumption and respiratory quotient were determined in premature infants maintained at neutral temperature. The effects on these parameters of intravenous infusion of 24 mg/kg/day carnitine were studied. Total, free and acylcarnitine increased and the acyl/free carnitine ratio decreased significantly during the four-hour study period. Resting heat production and respiratory quotient remained practically unchanged throughout the study period, indicating that in the face of carnitine sufficiency exogenous carnitine did not influence whole body heat production and substrate utilization pattern in premature infants. Further examinations in carnitine depleted infants will be required to clarify the regulatory role of carnitine in neonatal fatty acid metabolism and non-shivering thermogenesis.