The branchial elimination of pentachloroethane and four congeneric polychlorinated biphenyls by rainbow trout was measured using a fish respirometer-metabolism chamber and an adsorption resin column. Branchial elimination was characterized by calculating a set of apparent in vivo blood:water partition coefficients (P B W ). Linear regression was performed on the logarithms of P B W estimates and the logK O W value for each compound to give the fitted equation: logP B W =0.76.logK O W -1.0 (r 2 =0.98). The linear nature of this relationship provides support for existing models of chemical flux at fish gills and suggests that a near equilibrium condition was established between chemical in venous blood entering the gills, including dissolved and bound forms, and dissolved chemical in expired branchial water. In vivo P B W estimates were combined with P B W values determined in vitro for a set of lower logK O W compounds (Bertelson et al., Environ. Toxicol. Chem. 17 (1998) 1447-1455) to give the fitted relationship: logP B W =0.73.logK O W -0.88 (r 2 =0.98). The slope of this equation is consistent with the suggestion that chemical binding to non-lipid organic material contributes substantially to blood:water chemical partitioning. An equation based on the composition of trout blood (water content and the total amount of organic material) was then derived to predict blood:water partitioning for compounds with logK O W values ranging from 0 to 8: logP B W =log[(10 0 . 7 3 l o g K o w .0.16)+0.84].