The effects of increasing rates of Pb, Zn and Cu on extractable heavy metal levels, barley yields, basal respiration and the activities of catalase, urease, invertase and acid phosphatase were investigated in 2 soils in a 2-year greenhouse experiment. In the first year, barley yields were decreased by increasing additions of Pb, Zn and Cu. In the second year, increased yields were recorded at lower rates of addition of all 3 metals in both the Chestnut and Chernozem soils. Yield depressions were most marked for added Cu and least for Pb but, in contrast, accumulation of heavy metals in grain, in excess of recommended limits, was most pronounced for Pb and did not occur for Cu. Increasing rates of all 3 metals caused a decrease in basal respiration; the degree of inhibition was generally greater in the second than first year. After 1 year of incubation, increasing rates of addition of metals caused a marked reduction in the activity of all 4 assayed enzymes. However, after 2 years, the pattern of response was more complex with increases in enzyme activities being noted at lower rates of addition of all 3 metals. In general, invertase and urease activities were more markedly inhibited by heavy metal contamination than those for catalase or phosphatase. Ammonium acetate-extractable heavy metals in soils were less after 2 than 1 years reaction time due to their transformation into less labile forms. Nevertheless, negative correlations of grain yield, basal respiration and enzyme activities with heavy metal contamination were similar in each year whether extractable or total concentrations of Pb, Zn and Cu in the soil were used.