Aquatic Toxicology, 25 (1993) 1 14 I © 1993 Elsevier Science Publishers B.V. All rights reserved 0166-445X/93/$06.00 AQTOX 00556 The influence of temperature on the uptake rate constants of hydrophobic compounds determined by the isolated perfused gills of rainbow trout ( Oncorhynchus mykiss) Dick T.H.M. Sijm a, Peter PS.rt b and Antoon Opperhuizen a ~Research Institute of Toxicology. Environmental Chemistry Group. University of Utrecht. Utrecht. The Netherlands," bDepartment of Zoophysiology, Uppsala University, Uppsala. Sweden (Received 25 October 1991; revision received 15 August 1992:accepted 7 November 1992) Uptake rate constants of four hydrophobic and one less hydrophobic compound were determined using the isolated perfused gills of rainbow trout. Uptake rate constants thus obtained were in accordance with in vivo literature data. The four hydrophobic compounds (log Kd.,~ t > 3) had similar uptake rate constants. The fifth, less hydrophobic compound, phenol in undissociated form (pH <- pK~) had a lower uptake rate constant. The uptake rate constants were reduced by lowering the temperature below the acclimation temperature of the fish. The temperature dependent decrease was most pronounced for hexabromoben- zene, probably due to steric effects. The rate-limiting step in uptake seemed to be diffusion, either through the aqueous diffusion layer or through the lipid membrane, and not ventilation or blood flow-rate. Key words: Bioaccumulation; Uptake; Membrane permeation; Rate limitation INTRODUCTION The uptake process of chemicals in fish is of considerable interest in toxicological studies (Erickson and McKim, 1990a,b; McKim and Erickson, 1991). Hydrophobic xenobiotics as well as respiratory gases have low solubilities in water and are normal- ly taken up via the gills by passive diffusion (Randall et al., 1991). After uptake, hydrophobic compounds are eliminated slowly from the fish because of their affinity for fish lipids. Correspondence to." D.T.H.M. Sijm, Research Institute of Toxicology, Environmental Chemistry Group,: University of Utrecht, P.O. Box 80.058, NL-3508 TB Utrecht, The Netherlands. Tel.: 31-30-532316; Fax: 31-30-532837.