The outcome of intestinal infection with rotaviruses is more complex than initially appreciated, and it is affected by a complex interplay of host and viral factors. Rotaviruses infect intestinal enterocytes, and the early events in infection are mediated by virus-epithelial cell interactions. Diarrhoea may be caused by several mechanisms including (i) malabsorption that occurs secondary to the destruction of enterocytes, (ii) villus ischaemia and activation of the enteric nervous system that may be evoked by release of a vasoactive agent from infected epithelial cells in the absence of significant pathologic lesions or enterocyte damage, and (iii) intestinal secretion stimulated by the intracellular or extracellular action of the rotavirus non-structural protein, NSP4, a novel enterotoxin and secretory agonist with pleiotropic properties. New studies of rotavirus infection of polarized intestinal epithelial cells show that rotaviruses infect cells differently depending on whether or not they require sialic acid for initial binding, and infection alters epithelial cell functions. NSP4 also affects epithelial cell function and interactions. NSP4 (i) induces an age- and dose-dependent diarrhoeal response in young rodents that is similar to virus-induced disease, (ii) stimulates a Ca(2+)-dependent cell permeability where the secretory response is age-dependent, and (iii) alters epithelial cell integrity. Antibody to NSP4 protects mouse pups from diarrhoea induced by homotypic and heterotypic viruses. These data support a new mechanism of rotavirus-induced diarrhoea whereby a viral enterotoxin triggers a signal transduction pathway that alters epithelial cell permeability and chloride secretion. This new information about how a gastrointestinal virus causes disease demonstrates common pathogenic mechanisms for viral and bacterial pathogens not previously appreciated. These results also suggest new approaches to prevent or treat rotavirus-induced diarrhoea.