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. Keywords-Bioconcentration DiffusionConvective mass transport Gill morphometry Modeling CHEMICAL EXCHANGE ACROSS GILL MEMBRANESTo characterize the exchange of a nonpolar, nonmetabolized organic chemical across a fish's gills as a diffusion process, the fish's total body burden, Bf = mass/fish, of the chemical is described simply by = SJ, dB' dt where S is the fish's total gill area and Jf is the diffusive flux per unit area of gill surface. According to Fick's first law, the steady-state diffusive flux between two points, zl and z2, without a phase change is given by where J is the mass flux per unit area normal to the direction of diffusion (e.g., mass cm-' s -I ) , D is the solute's diffusion coefficient (e.g., cm2 545 546 M. C. BARBER ET AL. where k, = D , / h , is the toxicant's conductivity through gill epithelium and &, and C, are the concentrations of toxicant at the water-epithelial and the epithelial-capillary interfaces, respectively. An expression for Jf can be formulated using the steady-state relationship Jf = Jw = J, and assuming that: B,/V= Cf = PaCa + PICI f PsCs = (Pa + PIC,/Ca + PsCs/Ca)Ca where Vis the fish's volume (i.e., cm3 = grams for neutrally buoyant fish) and Cf is the concentration of toxicant in the fish's whole body. Moreover, if internal distribution of the toxicant is rapid in comparison to its kinetic uptake and elimination, the above equation can be simplified to A l . The relevant concentration gradient for exchange is B , / V = Cf = (Pa + PIK, + PsKs)Ca (8) between the interlamellar water and the aqueous portion of the capillary blood.