The effect of cholera toxin (CT) and Escherichia coli heat-stable enterotoxin (ST) on the ileum and colon was examined in vivo in the rat in an attempt to clarify the effects of enterotoxins on colonic mucosa and to determine if these effects were influenced by short-chain fatty acids (SCFA). Both CT and ST induced similar changes in water and elec trolyte fluxes, and the magnitude of these changes in loops of colon was similar to that observed in loops of ileum. The addition of luminal SCFA,

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... etate, propionate and butyrate did not influence the effect of either toxin in loops of ileum. However, in loops of colon exposed to CT, luminal butyrate (40 mM ) largely reversed the effect of CT by converting net water secretion (mean ± SE, -363 ± 154 nTcm"2-min_l) to net water absorption (470 ± 194 nl-cm-2-min_1) and by significantly reducing the net secretion of sodium ions. In loops of colon exposed to ST, similar effects were observed although net water secretion (-784 ± 114 nl-cm-2-min-') was only partially reversed by butyrate (-318 ± 102 nl"cm-2*min-1). In contrast to butyrate, acetate and propionate did not influence changes in colonic fluxes of water and sodium induced by enterotoxins. Oxidation of butyrate and glucose was observed to be depressed in colonocytes pre-exposed to CT but not to ST. In this model, colonic secretion induced by enterotoxins is similar to that observed in the ileum but differs from ileal secretion in its modulation by luminal butyrate. Diarrhea caused by cholera and entero toxigenic Escherichia coli is a major cause of morbidity and childhood mortality in devel oping countries [1] despite the benefits achieved by oral rehydration solutions. In health, the colon plays a significant role in the conservation of water and sodium ions [2] , but the effect of bacterial enterotoxins