A model describing thermodynamically driven kinetic exchange of organic chemicals between fish and the aqueous environment is developed. This model considers both the biological attributes of the fish and the physicochemical properties of the chemical that determine diffusive exchange across gill membranes. Important biological characteristics addressed by the model are the fish's gill morphometry, body weight, and fractional aqueous, lipid and structural organic composition. Relevant physicochemical properties are the chemical's aqueous diffusivity, molar volume, and n-octanol/water partition coefficient (Kow), which is used as a surrogate to quantify chemical partitioning to the fish's lipid and structural organic fractions. Using this model, excretion rates, gill uptake efficiencies, and bioconcentration factors can be predicted for nonmetabolized organic chemicals.