Carnitine is a natural substance that acts as a carrier of fatty-acids across the inner mitochondrial membrane for subsequent beta-oxidation. Acetyl-L-carnitine is the acetyl derivative of L-carnitine that has been shown to possess a slight cholinomimetic activity. Its success in sports medicine is dependent on the fact that it is able to stimulate the central nervous system functions. This study aims to investigate the effects of L-carnitine (LC) and its derivatives-acetyl-L-carnitine (ALC) and propionyl-L-carnitine (PLC)-on gastric acid secretion in rats. A concentration-dependent relationship with both ALC or PLC was observed in experiments in vitro using a rat isolated stomach. The addition of atropine to the perfusion bath only partially antagonized the effects of the two compounds. Stimulation of gastric acid secretion in a dose-dependent manner was also found when the tested compounds were administered i.v. to anaesthetized rats. To elucidate the mechanism of the gastric secretory response, assay for acetylcholine esterase activity using acetylthiocholine as substrate, was performed. It was found that ALC and PLC inhibited acetylcholine esterase, however, the IC50 for both compounds was about four times of magnitude greater than that of eserine. As the increase of the gastric acid secretion promoted by carnitines was blocked only partially by atropine both in vitro and in vivo, whilst it was completely abolished by experimental degeneration of the sympathetic neurons or by blockade of the postsynaptic sympathetic receptors, it is suggested that the effect of carnitines is determined by cholinergic and partly by adrenergic mechanisms.