The gill fluxes of various non-electrolytes were measured in fresh-water- and sea-water-adapted trout (Salmo gairdneri). The studies were performed in vitro with a 'perfused-head' preparation. 2. The results allow one to specify different transepithelial pathways according to the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic and lipophilic molecules of small molecular radius, a transcellular pathway in the respiratory cells of the secondary lamellae, (2) for hexose, all paracellular pathways in the gill epithelium, and (3) for hexose polymers (inulin, dextran), a transcellular pathway in the chloride cells of the primary lamellae. 3. The selectivity of the respiratory cells as a function of the liposolubility of the molecules tested is low. The decrease of this selectivity in the course of salt-water adaptation taken together with the modification of lipid composition of membranes and the effect of adrenaline on the branchial permeability suggests that non-electrolytes diffuse through a lipid phase able to form hydrogen bonds. 4. The high permeability of gills to hexose polymers of high molecular weight suggests a vesicular transport, especially in fish adapted to fresh water.
