A model is presented describing the kinetics of uptake and release of nonmetabolizing organic chemicals by fish from water. The model contains three parameters: a bioconcentration factor which is specific to the chemical (and can be characterized by the octanol/water partition coefficient and fish lipid content), a water-phase resistance term and a lipid-phase resistance term which are specific to the fish. These parameters can be estimated from uptake-clearance experiments. Part of the water-phase resistance term can be attributed to gill ventilation rate as estimated from gill uptake efficiencies and part is an “internal” resistance. Procedures are suggested for scaling these parameters to different fish sizes. The dependence of these resistances, and uptake and clearance rate constants, on chemical hydrophobicity is discussed and quantified.