The influence of Cu²⁺ toxicity on dry mass, chl.a, carotenoids, protein content, Cu²⁺ uptake, activities of some enzymes of nitrogen assimilation and alkaline phosphatase in the diazotrophic cyanobacterium Anabaena variabilis was evaluated.Lower Cu²⁺concentrations stimulated the dry weight, chl.a, and protein content, whereas, higher concentrations were inhibitory. In contrast, a prominent increase in the carotenoids content was detected in response to all Cu²⁺ treatments. A. variabilis showed high uptake capacity of Cu²⁺. The accumulation capacity is directly proportional to the external Cu²⁺ concentrations. A stimulation in the activities of glutamine synthetase (GS), NADH-glutamate synthetase (GOGAT), nitrate reductase (NR), nitrogenase and alkaline phosphatase (AP) in response to low Cu²⁺  concentrations was noted. However, higher ones inhibited the enzymes activities with different degrees. The enzymes could be arranged according to their tolerance to Cu²⁺ toxicity in the following order:  AP > NR > GOGAT > GS > nitrogenase.