Using specimens of human submandibular glands, we have investigated in vitro the morphological modifications induced by clozapine, a dibenzodiazepine derivative that is used in psychotic patients and that provokes hypersalivation, a side-effect of therapy. The effects of the drug, used alone or in combination with carbachol, have been compared with those observed after treatment with drugs acting on specific receptors. To quantify the response to stimulation, we have calculated (with statistical methods) the number of microvilli and microbuds (corresponding to pits seen in images obtained by transmission electron microscopy) per square micrometre of the cytoplasmic surface of the intercellular canaliculi luminal membrane in images obtained by high-resolution scanning electron microscopy. Clozapine, when directly acting on human submandibular specimens, induces a small secretory response in serous cells; this is partially decreased by muscarinic and adrenergic antagonists and by combined incubation with carbachol, thus confirming its behaviour as a partial agonist to muscarinic receptors. We also suggests that the drug acts on the nerve terminals contained within the glandular specimens.