Food and Gill Exchange of Toxic Substances (FGETS) is a FORTRAN simulation model that predicts temporal dynamics of fish whole body concentration (ug chemical/(g live weight fish)) of non ionic, non metabolized, organic chemicals that are bioaccumulated from either: (a) water only - the predominant route of exchange during acute exposures, or (b) water and food jointly - characteristic of chronic exposures. FGETS calculates time to reach the chemical's lethal activity by assuming the chemical elicits its pharmacological response through a narcotic mode of action. FGETS considers both the biological attributes of the fish and the physico-chemical properties of the chemical that determine diffusive exchange across gill membranes and intestinal mucosa. Important biological characteristics addressed by model are the fish's gill and intestinal morphometry, the body weight of the fish, and fractional aqueous, lipid, and structural organic composition.