Abstract 1. 1. The effects of 2,4,6-triamino-pyrimidine (TAP 10 −2M) and of diphenylamine-2-carboxylate (DPC 10 −4M) on the transepithelial potential difference in isolated, perfused preparations of anterior and posterior gills of the Chinese crab have been studied. 2. 2. 2,4,6-triamino-pyrimidine (TAP) added either IN or OUT, depolarizes the anterior gills and hyperpolarizes the posterior ones. These effects are interpreted considering (a) an inhibition of paracellular Na + movements in the anterior gills and (b) an inhibition of the serosal K + outward diffusion in both the anterior and the posterior gills. 3. 3. Diphenylamine-2-carboxylate (DPC) is effective only on the posterior gills when added to the perfusion saline. It then induces an important depolarization, consistent with an inhibition of Cl − channels at the serosal side of the epithelium.