Metal uptake and its effect on foliar metallothionein 2b (MT2b) mRNA levels were studied in hybrid aspen (Populus tremulaxtremuloides) in field conditions. The trees were planted on a site contaminated with several metals, including cadmium (mean 5.1 mgkg(-1)), chromium (80 mgkg(-1)), copper (180 mgkg(-1)), nickel (81 mgkg(-1)), vanadium (240 mgkg(-1)) and zinc (520 mgkg(-1)). Of the ten trace elements analyzed, only Cd and Zn accumulated in the leaves with maximal foliar concentrations of 35 and 2400 mgkg(-1) (dry weight), respectively. There was a strong correlation between Cd and Zn concentrations and bioaccumulation factors (concentration in plant/soil) in the leaves, branches and roots, suggesting similar transport mechanisms for these two metals. The levels of MT2b correlated with Cd and Zn concentrations in the leaves, demonstrating that increased MT2b expression is one of the responses of hybrid aspen to chronic metal exposure.