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Micro-electrodes sensitive to Cl- have been used to measure Cl- activities in the mucous layer on the mucosal surface of an isolated oesophageal preparation of the fresh-water teleost, Phoxinus phoxinus (L.). The technique has also been used to estimate the diffusion coefficient of Cl- in mucus in situ on the mucosal surface of this preparation. The mucous layer supports a linear gradient of Cl- which represents approximately 20% of the gradient of Cl- between mucosal and serosal solutions. The estimate of the diffusion coefficient of Cl- in mucus was 0.30 X 10(-5) +/- 0.029 X 10(-5) cm2 s-1 (mean +/- S.E. of mean, n = 13). This value is approximately 20% of published values of the diffusion coefficient in pure NaCl solutions or in teleost saline. It is suggested that the mucous layer acts to lower the permeability of the oesophagus in two ways. Its viscous and gel-forming nature encourages the formation of an unstirred layer, through which diffusion is the only means whereby salts can move. Also, it provides a layer through which Cl- and possibly other ions diffuse much more slowly than they would through a similar layer of mucosal solution without mucus.