Increased amounts of heavy metals (Cd and Zn) adversely influenced the growth and productivity of water plants. Cadmium (Cd) is a non-essential and toxic element, whereas zinc (Zn) is an essential element required for activity of many enzymes, and plays a significant structural role as stabilizer of proteins and membranes. The objective of this study was to estimate the interactive effects of Cd and Zn in duckweed, Lemna minor L. - widely spread monocot often used as a model organism. Cadmium, (10 μM) and zinc (100 μM and 200 μM) and also their combination (10 μM Cd + Zn 100 μM and 10 μM Cd + Zn 200 μM) were added in the nutrient medium for cultivation of duckweed. The plant growth as well as antioxidative enzymes catalase (CAT), ascorbat-peroxidase (APX) and pirogalol-peroxidase (POX) activities were measured. The significant growth reduction and chlorosis were observed in the plants treated with 10 μM Cd already at the sixth day of the experiment. Under the influence of investigated metals the enzymes catalase and peroxidases have shown different levels of activity. The third day of the experiment plants treated with cadmium had very high activity of CAT, while in the samples treated with both metals, Cd and Zn, the increase was not so obvious. While APOX activity was increased in the certain treatments at the sixth day of the experiment, POX activity was not significantly changed. Presented results shown that cadmium causes oxidative stress in plant which leads to activation of antioxidative enzymes, primarily catalase. Zinc, and combinations of zinc and cadmium also cause oxidative stress and induce activity of antioxidative enzymes, but to a lesser extent than cadmium. Cink added into nutrient medium containing cadmium, can to a certain extent alleviate toxic effect of kadmium.