SUMMARY

1. Water, potassium and sodium fluxes were measured across various regions of the locust gut under in vitro internal perfusion. 2. The anterior caeca differed from the other gut regions, notably the posterior caeca, in being able to absorb water in the absence of an applied chemical or osmotic gradient. 3. Absorption of sodium from the caecal lumen was active, and inhibited by 2,4-DNP or ouabain. 4. Potassium was passively distributed across the caecal epithelium, and transmembrane fluxes were unaffected by toxins. 5. A model for caecal water uptake was advanced, in which passive fluxes of potassium and chloride from the KCl-rich luminal fluid, combined with active sodium uptake, drives water into the blood, thus concentrating solutes within the caecal lumen