Bicycle ergometer exercise tests were used to study two groups of 10 subjects suffering from type A chronic respiratory insufficiency before and after a 4-week course of L-carnitine or placebo. Analysis of the ventilatory and metabolic response to physical exercise (60 watts) revealed an improved utilization of the bio-energetic substrates obtained with L-carnitine as compared to patients treated with placebo. The substantial change observed in ventilatory and metabolic response to exercise after L-carnitine is characterized by an appreciable production of lactate and by the stability of pCO2, pH and HCO3 values, which would appear to support the hypothesis of a reduced anaerobic metabolism requirement for energy purposes.