The effect of glibenclamide, a chloride and potassium channel blocker, clotrimazole, a potassium channel blocker and cromokalim, a potassium channel opener were tested in vivo for their ability to prevent E.coli STa enterotoxin from inhibiting jejunal fluid absorption in the anaesthetised rat. Potassium channel opening is thought to play a central role in the maintenance of the enterocyte membrane potential that provides the correctly aligned electrochemical force needed to promote chloride ion secretion towards the intestinal lumen. Sections of ileum from rabbits post mortem were mounted in a Burn-Dale apparatus and longitudinal muscle tension was measured. The ability of these compounds to restore STa inhibited fluid absorption was examined. A perfused intestinal loop preparation was used to measured luminal uptake of fluid in vivo by volume recovery from the jejunum of the anaesthetised rat (70 mg/kg i.p. Sagatal). All procedures were carried out in conformity with UK legislation. Data are given as the mean plus the standard error of the mean with the number of loop experiments in parenthesis. Significance was tested by ‘t’-test. The unchallenged jejunum absorbed fluid at a rate of 98.8 ± 6 (8) ul/cm/hr. In the STa (80 ng/ml) challenged loop, there was a significantly lesser (p<0.001) rate of 33.2. ± 8.1 (9) ul/cm/hr absorption, that was also significantly higher (p< 0.001) than zero absorption. Intraluminal 17 uM glibenclamide plus E.coli STa gave a rate of fluid absorption of 24.8 ± 8.9 (6) ul/cm/hr. Clotrimazole at 10 uM and STa gave a rate of fluid absorption of 17.0 ± 8.4 (5) ul/cm/hr. Intraluminal cromokalim of 1 uM plus STa gave a rate of 43.9 ± 11.9 (6) ul/cm/hr., not significantly different from values for STa challenge in the absence of cromakalim. Clotrimazole, glibenclamide and cromakalim did have significant effects on the cardiovascular system in vivo when given as an i.v bolus dose and also on rabbit ileum motility in vitro. Intravenous glibenclamide (250ug per 15 minutes) and clotrimazole (300 ug per 15 minutes) also failed to restore STa challenged fluid absorption. Cromakalim (10 ug per 15 minutes) worsened (p<0.05) STa inhibited fluid absorption to 10.3 ± 6.8 (5) ul/cm/hr. These compounds were identified as anti-secretory agents in vitro on the basis of their ability to reduce short-circuit current elevation after STa exposure back to pre-challenge levels. When tested in vivo, these compounds are unable to prevent the reduction in fluid absorption after STa challenge. These findings contradict the concept of electrogenic chloride ion secretion as the basis for diarrhoeal disease, since the pharmacology of inhibition of elevated short circuit current is not matched by an ability to restore STa challenged fluid absorption.