Cu-64 and,, total Cu accumulation were measured in gills, plasma, liver, kidney, bile and urine during 72 h of exposure to Cu-64, at 20 mu g Cu l(-1), in non-acclimated and Cu-acclimated (28 days of pre-exposure) rainbow trout (Oncorhynchus mykiss) fitted with urinary bladder catheters. Renal Cu excretion gradually declined from 0.03 mu g Cu kg(-1) h(-1) in non-exposed fish to 0.01 mu g Cu kg(-1) h(-1) after 28 days of Cu exposure. A comparison of the Cu-64-labelled Cu and the total Cu excretion rates and the corresponding renal clearance revealed apparent differences in Cu binding to plasma protein depending on whether the Cu is derived from recent branchial uptake or is already present in the plasma prior to Cu-64 exposure. The plasma Cu pool derived from recent branchial uptake and the Cu pool present in the plasma prior to Cu-64 exposure is accessible to renal excretion to different extents, whereas the pools seem equally accessible to hepatic accumulation and elimination. The renal Cu excretion is of minor importance compared with the hepatic Cu excretion, which was estimated to be 0.5-0.75 mu g Cu kg(-1) h(-1) and 1.1-1.6 mu g Cu kg(-1) h(-1) for non-acclimated and Cu-acclimated fish, respectively. Based on the biliary Cu concentration, hepatic Cu elimination appeared to be stimulated in the Cu-acclimated relative to the non-acclimated fish. Only 17% and 12% of the hepatic Cu could be accounted for by metallothionein in the control and Cu-acclimated fish, respectively. Renal Na+ efflux decreased by 40%, which was largely due to increased tubular Na+ reabsorption. Renal compensation for the impaired branchial Na+ uptake, seen during Cu exposure, thus seems to be involved in Cu acclimation in rainbow trout. (C) 1998 Elsevier Science B.V.