The steady-state fluxes of K + , Cl - and Na + from smooth muscles in normal Krebs solution are described, and some of the problems encountered in the interpretation of such results are discussed. Sodium fluxes are particularly difficult to analyse, and the type of model used to estimate sodium permeability from flux curves, determines the value calculated to a large extent. In order to simplify the ionic distribution in the tissue, in the hope of obtaining more information about the handling of Na and K by the guinea-pig taenia coli, potassium-free (high Na) tissues and sodium-free (high K) tissues were prepared by soaking for 4 h in the relevant solution. The tissues reach a steady state, and the effluxes of 42 K from high K tissues and 24 Na from high Na tissues were measured and compared. In these two conditions the electrochemical gradients for Na + and for K + are probably identical. Nevertheless, the fluxes are markedly different. At 37 °C the Na flux is much faster than the K flux, and a part of the Na exchange is very temperature sensitive. There is also some evidence for a sodium calcium exchange mechanism. It is hoped that further experiments will produce results that will help to understand the processes involved in these exchanges.