The actions of cimetidine are well known: an inhibitor of gastrin hydrochloric secretion, cimetidine modifies neither the motility nor any of the other functional factors. Numerous authors also attribute a cytoprotective role to cimetidine, one that is apparently carried out independently of the inhibitory effect on hydrochloric secretion. The cytoprotective mechanism itself is not yet fully understood. Numerous hypotheses have been made, including increased production of bicarbonates, inhibition of cyclic AMP, increased mucosal blood flow, and increased mucous production. However, all these hypotheses have not been fully explained, hence the problem remains open to further contributions. Well accepted, instead, is the possibility that the drug may not only intervene in acid-hydrochloric inhibition but may also, by broadening its mechanisms, enhance the powers of the mucosal barrier.