Oxmetidine, a new and more potent analogue of the H2 receptor antagonist, cimetidine, was recently withdrawn from clinical trials because of associated hepatotoxicity. We investigated the potential hepatotoxicity of the drug in vitro and in vivo in the rat. In addition, we investigated, in in vitro experiments, the potential hepatoxicity of other gastric acid inhibitory drugs (cimetidine, ranitidine, omeprazole and nolinium bromide). In in vitro experiments, oxmetidine, at various concentrations, was added to isolated hepatocyte incubations and cytoxicity was assayed by trypan blue exclusion. In in vivo experiments, oxmetidine was administered both i.p. and orally, and hepatotoxicity was assessed by serum biochemical measures (transaminases, alkaline phosphatase, S'nucleotidase, gamma glutamyl transpeptidase) and liver histopathology. In the in vitro studies, the addition of oxmetidine to the hepatocyte incubations was associated with significant (P<O.OOl) dose and time dependent cytotoxicity . However, the in vivo experiments revealed no significant changes in serum biochemistry and no significant alterations in liver histopathology up to 72 h following the administration three different dosages of oxmetidine. Of the other gastric acid inhibitory drugs, only nolinium bromide was associated with significant (P<O.OOl) in vitro cytotoxicity. Our in vitro observations establish that oxmetidine is cytotoxic to isolated rat hepatocytes and suggest that nolinium bromide be further evaluated for potential hepatotoxicity.