The use of H₂-receptor agonists and antagonists allowed us to establish that histamine H₂-receptors are present in pancreatic exocrine tissue and their stimulation caused a dose-dependent increase in pancreatic juice. The fact that H₂-antagonists from one side and aminoguanidine from the other were unable to modify basal levels of pancreatic secretion, seems to minimize a role for H₂-receptors in the regulation of pancreatic secretion. On the other hand H₂-antagonists modified ceruletide-induced secretion in different ways according to the different molecules. Ranitidine strongly potentiated whereas cimetidine, oxmetidine and mifentidine slightly inhibited the effect of ceruletide. The stimulatory effect of eserine and the inhibitory effect of atropine indicate a cholinergic interference in the action of ceruletide. Therefore the potentiating effect of ranitidine may be related to its cholinomimetic action and the inhibitory effect of the other H₂-antagonists may be connected with an anticholinergic effect. However, the potentiating effect of aminoguanidine on ceruletideinduced secretion may indicate a possible role for histamine in the response to ceruletide.