Uptake rate constants of different classes of hydrophobic organic chemicals have been determined in isolated perfused gills of rainbow trout (Oncorhynchus mykiss) as an alternative for studies in vivo. The uptake rate constants have been compared to those determined in guppy, Poecilia reticulata, in vivo. The organic chemicals which have been used are anthracene, hexabromobenzene, octachloronaphthalene, octachlorodibenzo-p-dioxin, phenol, polychlorinated anisoles, polychlorinated benzenes, polychlorinated biphenyls, and tetrachloroveratrole. Uptake rate constants in guppy are higher than those in rainbow trout gills, and show relatively high variation in both gills and guppy. When uptake rate constants in each study are normalized for that of pentachlorobenzene (pCBz), variation is significantly reduced both in perfused gills and in guppy. All allometric relationship is derived between weight and uptake rate constant. Uptake rate constants determined in one fish can thus be used for prediction of those in other fishes. When a reference chemical, such as pCBz, is included, the gill perfusion experiments can be highly suitable to determine uptake rate constants of organic chemicals, which can be extrapolated to fish of different sizes.