Ussing chambers were mounted with stripped rabbit ileal mucosa and either crude or pure toxin A added to the mucosal side of the chamber to final concentrations of 800 and 30pg/ml respectively; increases in short-circuit currents of the order of 25pA were observed in both preparations over a period of 90min, at which point the tissue was reponsive to theophylline but not glucose. Histology showed that damage to villus tips had occurred by 90min in contradistinction to Hughes et al. (1983) who claimed no damage was induced in similar experiments. Unidirectional flux experiments have not yet been carried out and may be of equivocal value in analysing perturbations in active transport systems until and unless effective means are found for controlling the effects due to cell damage.