The serotonin (5-HT) content of enterochromaffin cells (EC) was studied by a cytofluorimetric method in biopsies from rat duodenal mucosa after in vitro incubation with different adrenoceptor agonists and antagonists and acetylcholine (ACh). Noradrenaline (NA), Adrenaline (A) and Isoprenaline (IP) caused a decrease of 5-HT in EC down to 40--60% and for NA and A this effect was concentration-dependent. The effect was antagonized by d,1-propranolol but not by d-propranolol, metoprolol, phentolamine or phenoxybenzamine indicating that the 5-HT release from EC is probably mediated via a true beta-adrenoceptor mechanism possibly of the beta2 type. ACh also decreased the 5-HT content of EC but was much less potent than the adrenergic substances. Dopamine (DA) had no effect.