Melatonin occurs in large amounts in the intestinal mucosa and is released during a meal. Recent studies of ours reveal that exogenous melatonin evokes the in vivo secretion of protein and amylase from the rat parotid gland. The aim of the present study was to investigate the effect of melatonin on the protein synthesis of the parotid gland of pentobarbitone-anaesthetised rats as estimated by the rate of incorporation of [³H]leucine into trichloroacetic acid-insoluble material of the gland. Compared with the parotid protein synthesis (set at 100%) of those rats exposed to an intravenous infusion of melatonin (25 mg/kg during 1 hour), under muscarinic and α- and β-adrenoceptor blockade, the synthesis in the corresponding glands of saline-treated control rats was less (by 25%). The synthesis was also less when the melatonin administration was combined with the melatonin 2-preferring receptor antagonist luzindole (24%), the non-selective nitric oxide synthase inhibitor L-NAME (18%) and the neuronal nitric oxide synthase inhibitor N-PLA (21%). Almost all the melatonin receptor-mediated effect was due to nitric oxide generation via the activity of neuronal type nitric oxide synthase. The present findings lend further weight to the idea that salivary glandular activity associated with food intake is hormonally influenced and they also suggest clinical implications for melatonin in the treatment of xerostomia. Since melatonin is known to exert anti-inflammatory actions in the oral cavity, the stimulatory effect of melatonin may include the synthesis of proteins of importance for the oral defence.