1. Na and Cl intake through the gill of the perfused head of trout were studied in fresh water with (10(-5)M) or without adrenaline in the perfusing solution. 2. Ionic influxes occur exclusively across the lamellae in fresh water while in the sea-water adapted trout part of salt entry is extralamellar. 3. In absence of adrenaline, Na and Cl enter the gills at the same rate (respectively 6.9 +/- 1.30 and 6.6 +/- 1.55 muequiv/hr. 100 g). Adrenaline (10(-5)M) increased the Na influx to 47.8 +/- 4.12 muequiv/hr. 100 g, a value similar to that observed in vivo. The Cl influx remains unchanged however (6.3 +/- 2.40 muequiv/hr. 100 g), a value much smaller than that found in vivo. 4. Radioactive loading experiments coupled with unloading experiments allowed the determination of the relative permeabilities of the serosal and mucosal barrier for Na+ and Cl-. For both ions, the basal membrane is less permeable. Adrenaline by increasing the Na permeability across the apical barrier enhances the active Na pumping through the basal membrane. 5. Intracellular Na and Cl exchangeable pools were calculated. They represent less than 1% of the total ionic content of the epithelium. Adrenaline increased by sixfold the Na pool without modifying the Cl pool.