Intracisternal injection of TRH analogue, RX 77368, which has been previously shown to induce a vagal-dependent stimulation of gastric acid secretion and contractility dose-dependently (10-300 ng) stimulated serotonin release into the gastric lumen of conscious pylorus-ligated rats. The stimulation of gastric acid and serotonin release occurred 15 and 30 min, respectively, after intracisternal injection of RX 77368 (100 ng) and lasted for over 2 h, as measured in urethan-anesthetized rats with gastric fistula. Cervical vagotomy and atropine pretreatment (0.1 mg/kg) completely abolished the RX 77368-stimulated gastric serotonin and acid output in acute gastric fistula rats, whereas in pylorus-ligated rats atropine pretreatment markedly reduced RX 77368-stimulated gastric fluid volume and serotonin output, but not serotonin concentration. Systemic administration of bethanechol chloride, histamine, or pentagastrin, at doses that elicited gastric acid secretory responses similar to that of intracisternal TRH analogue, produced little or no intraluminal serotonin release. These results demonstrate that intracisternal injection of TRH analogue induced a marked and sustained release of serotonin into the gastric lumen that is mediated by vagal-dependent mechanisms unrelated to enhanced acid secretion. Activation of muscarinic receptors appears to be involved, but the relative ineffectiveness of bethanechol in this system suggests that (an)other vagally mediated neurochemical event(s) stimulated by intracisternal RX 77368 acts in conjunction with muscarinic receptor activation to produce intraluminal serotonin release. The functional significance of serotonin release into the gastric lumen in relation with intracisternal TRH analogue-induced alterations of gastric secretion, motility and blood flow, and lesion formation remains to be elucidated.