The aim of this study was to assess the in vitro stability of ranitidine to colonic bacteria by utilising a batch culture fermentation system to simulate the conditions of the colon. Three quantities of ranitidine, 100, 200 and 500 mg, in the form of the hydrochloride salt, were introduced into individual 100 ml fermenters consisting of buffer medium inoculated with freshly voided human faeces (10% w/v). Control experiments were also run in parallel using equivalent drug quantities in buffer medium without the presence of faeces. Samples were removed at pre-determined time intervals over a 24 h period and were subsequently analysed by high-performance liquid chromatography (HPLC) for drug concentration. A selection of the samples removed from the fermenters was also analysed by conventional UV spectroscopy and mass spectrometry. Subsequent to an initial dissolution phase in the fermentation system, a marked decline in ranitidine concentration was noted over time, thereby suggesting degradation and metabolism of the drug by colonic bacteria. No such decline in concentration was noted in the control buffer systems. The rate and extent of metabolism was rapid and complete within 12 and 24 h for the 100 mg and 200 mg samples, respectively, although the largest sample size, 500 mg, was only partly metabolised over the course of the experiment. UV and mass spectrometry analysis indicated that metabolism occurred via cleavage of an N-oxide bond within the molecule with the resultant loss of an oxygen atom, although further metabolic reactions are possible. Such metabolism may in part be responsible for the poor bioavailability of ranitidine from the colon.