1. The effects of theophylline (1 mmol/l) on the sodium transport (short-circuit current, Isc) and transepithelial conductance (Gtotal) through toad (Bufo viridis) and frog (Rana temporaria and Rana esculenta) skin were investigated. 2. In toad skin incubated with nitrate Ringer solution on the apical side, theophylline induced an increase in Isc similar to that in frog skin bathed with chloride Ringer solution. 3. The increase in Isc could be attributed to recruitment of sodium channels, without affecting the single-channel current. 4. Chloride-bathed toad skin responded to theophylline with a large increase in transepithelial conductance, in addition to the increased Isc. 5. Chloride replacement by nitrate eliminated the effect of theophylline on the conductance increase, but the Isc response was even larger. 6. The results are discussed in relation to the localization of the cellular chloride pathway to the mitochondria-rich cells.