Changes in transepithelial water and electrolyte transport as causative or contributing factors of the diarrhoea and constipation found associated with changes in thyroid physiology were studied. Albino Wistar rats were pharmacologically made either hypothyroid or hyperthyroid. After sacrifice, the small intestine was mounted in Ussing chambers in order to measure in vitro ion net fluxes under short-circuit conditions. Hypothyroid animals showed an increase in intestinal transit time, Cl- absorption (mainly due to an increment in its mucosal to serosal component) and residual ion flux (which is believed to represent HCO3- secretion) when compared with euthyroid animals. The hyperthyroid animals showed a decrease in Cl- mucosal to serosal transport. Furthermore, a significant correlation was found between serum L-thyroxine (T4) levels and both net Cl- transport (r = -0.74, P < 0.00001) and residual ion flux (r = -0.55, P < 0.005). These results indicate that the effect of T4 is firstly to inhibit Cl-/HCO3- anion exchange thereby influencing transepithelial flux transport and secondly to affect intestinal motility. Such inhibition was not found when T4 was acutely added to rat ileum, suggesting that the effect on electrolyte transport probably requires protein synthesis. In conclusion, the phenomenon observed in vitro could explain the clinical manifestations of constipation and diarrhoea in hypo- and hyper-thyroidism respectively.