In order to evaluate the impact of water-borne copper on acid-base regulation in fresh water rainbow trout, chronically cannulated fish were exposed to copper (0.6 mg 1(-1)), hypercapnia (water PCO2 of 6 mmHg) or a combination of copper and hypercapnia, while a fourth untreated group served as the control. Blood samples obtained at 0 h, 4 h and 24 h were analysed for acid-base status, ion concentrations and respiratory parameters. Tissue samples from caudal skeletal muscle, liver and gill filaments were examined for intracellular acid-base status, ion- and water contents, and copper concentration. Exposure to copper alone elicited a small extracellular metabolic alkalosis, no changes in arterial PO2, and a minor decrease in plasma ion concentrations. Hypercapnia alone increased arterial PCO2 from approximately 2 mmHg to 7.2 mmHg, but the extracellular respiratory acidosis present at 4 h was almost completely compensated at 24 h due to an increase in plasma bicarbonate concentration [HCO3-] from 8.1 mM to 24.4 mM. Combined exposure to hypercapnia and copper resulted in a slightly larger acidosis at 4 h, and the fish failed to restore extracellular pH at 24 h, because plasma [HCO3-] only increased to 16.3 mM. Fish exposed to hypercapnia and copper also showed a delayed recovery of intracellular pH in skeletal muscle, compared to fish exposure to hypercapnia only. Thus, copper exposure impaired both extracellular and intracellular acid-base regulation during hypercapnia. When seen in connection with only minor effects of copper on osmoregulatory and respiratory parameters, the reduced ability to regulate acid-base suggests that acid-base regulation may be one of the most copper-sensitive branchial functions.