Plasma ions and cortisol levels were studied in immature tilapia for 6 days to a range of sublethal concentrations of Cu (50, 100 and 200 pg Cu I-1), Cd (20, 35 and 70 p g Cd I-1) and to combinations of these metals (50/ig Cu I-1 + 2 0 /*g Cd I ' 1, 100/ig Cu I-1 + 35 /ig Cd I-1 and 200 p g Cu I" 1 -f 70 p g Cd 1 " 1 ). Our data show that Na and Ca were markedly, although not exclusively, affected by Cu and Cd, respectively. Plasma Na concentrations were most prominently decreased in Cu-exposed fish, with less pronounced effects in Cd-exposed fish. In fish exposed to 10 pg Cd I-1, the plasma Ca concentration was half of the control value. Cu-induced changes of plasma Ca concentrations were less strongly marked. In combined Cu/Cd exposed fish, Na, Ca and Cl concentrations were significantly changed. Important in the present study was the notion that, in combined Cu/Cd exposed fish, the changes in Na and Ca levels could not be explained by synergism of addition of the effects observed in single metal exposed fish. Plasma cortisol levels were increased in Cu-exposed fish, but an increase was not observed in the Cu/Cd co-exposed fish. It is argued that the absence of this cortisol response contributes to an inadequate recovery of ionic disturbances in the Cu/Cd co-exposed fish.