1. Net fluid transport rate, transepithelial ohmic resistance and potential difference (p.d.), and unidirectional fluxes of Na+ were measured in rabbit gall-bladder preparations in vitro exposed on both sides to Ringer solutions of identical electrolyte composition. 2. Bilateral application of 1% dimethylsulphoxide (DMSO), the solvent for cytochalasin B, rapidly and reversibly depressed net fluid transport rate by 15% and increased the lumen positive p.d. by 1-5-2-0 mV. Resistance did not change significantly. These effects of DMSO were shown to be non-specific osmotic effects. 3. Cytochalasin B (10(-5)M) applied bilaterally caused: (a) a progressive inhibition of net fluid transfer rate to 40-50% of its control value within 60 min; the effect was partly reversible within 60 min and independent of the substrates glucose, glutamate and pyruvate; (b) a progressive depression of the mucosal-to-serosal Na+ flux within the first 30 min with no further change in the flux during the following 30 min of exposure to cytochalasin B; the effect was partly reversible within 70 min; (c) a rapid but moderate increase in the passive serosal-to-mucosal Na+ flux, which continued to increase gradually during the entire 60 min period of exposure to cytochalasin B; the effect was completely reversible within 70 min; (d) a prompt drop in ohmic resistance (30%) and p.d. (40%) with no further changes in these parameters during the following 60 min of exposure to cytochalasin B. The effect on resistance was partly reversible within 90 min; the effect on p.d. was completely reversible within 30 min. 4. The results are interpreted to indicate an early inhibitory action of cytochalasin B on the active transcellular pump mechanism and to suggest a cytochalasin B-mediated progressive increase in cell membrane permeability to sodium resulting ultimately in a highly leaky epithelium. The results are compatible with the concept that a mechanochemical process is involved in isosmotic transcellular transport of fluid across low-resistance epithelia.