The effects of MgCl2 on the electrical characteristics and sodium and chloride transport in short-circuited rabbit terminal ileum in vitro were studied. Increasing the magnesium concentration from a basal concentration of 1.1 mmol/l to either 2.5 or 10.3 mmol/l (mucosal or serosal addition) resulted in increases in the unidirectional flux of chloride from serosa to mucosa (JCl sm), short-circuit current (SCC) and transmucosal potential difference (p.d.). Increasing the magnesium concentration from a low basal concentration (0.3 mmol/l) to 10.3 mmol/l (mucosal or serosal addition) abolished net sodium absorption and converted net chloride absorption into net secretion, as a result of a decrease in the unidirectional flux of sodium from mucosa to serosa (JNa ms), and increases in the unidirectional fluxes of sodium and chloride from serosa to mucosa (JNa sm and JCl sm). Increases in SCC and p.d. occurred after mucosal, but not serosal, addition of magnesium. Incubation of stripped ileal mucosa with MgCl2 (10.3 mmol/l) did not result in increased mucosal concentrations of cAMP or cGMP. These data suggest that at least part of the cathartic effect of magnesium salts is likely to result from marked changes in ileal ion transport and provide a rational explanation for the watery diarrhoea commonly seen after the oral administration of magnesium salts to hypomagnesaemic patients.