In rabbit ileum in vitro Clostridium difficile toxin (200 microliter crude extract) almost abolished net Na absorption, by decreasing mucosa to serosa flux, and induced net Cl secretion by increasing the serosa to mucosa flux. These flux changes were induced when there was no visible histological damage to the mucosa. The toxin did not influence adenylate or guanylate cyclase activity in a plasma membrane fraction of isolated rabbit enterocytes nor did it affect cAMP concentrations in intact rabbit ileum pre-incubated with toxin. The flux responses to the toxin were prevented by removing calcium from the serosal medium, suggesting that the secretory process may be calcium dependent. These results indicate a possible mechanism by which this toxin could induce diarrhoea.