A review of the current state of knowledge and speculation on fluid absorption by the gastro-intestinal tract of vertebrates and more specifically the role of hormones in fluid movement has been made. The effect of hormones on water transport, active sodium transport and the electrical characteristics of poikilotherm vertebrate, bird and mammal intestine, both in vivo and in vitro are considered. Special reference is made to the effect of aldosterone on the absorption of fluid. Data indicate that while aldosterone has an effect on other tissues such as amphibian bladder, it appears to have only a slight effect on the jejunum and only on that component of the transport phenomena associated with the transfer of sodium.