Sycamore suspension cells (Acer pseudoplatanus L.) were used to investigate the effect of copper on respiratory electron transport. Alternative oxidase (AOX) protein content and enzymatic capacity increased as a function of the concentration of copper added to the culture medium, from 0.2 to 50 μM. The latter sublethal concentration, which arrests mitochondrial biogenesis and thereby decreases cell respiration rates, stimulated cyanide-insensitive oxygen uptake in cells and mitochondria isolated therefrom. This was correlated with the accumulation of two proteins (30 and 36 kDa) which reacted with monoclonal antibodies against AOX. An accumulation of a 1.6-kb AOX mRNA was also observed. The possible mechanisms through which copper affects AOX are discussed.