The toxic effects of chromium and tin salts on growth, 14C uptake, heterocyst differentiation, and nitrate reductase, nitrogenase, and glutamine synthetase activities of Anabaena doliolum and their regulation by pH, salinity, extracellular metabolites (spent), and organic acids have been studied. The toxicity of the test metals was lowered at alkaline pH and increased at acidic pH. NaCl at 20 mM was found to decrease metal toxicity. Extracellular metabolites (spent) in a 1:1 ratio (v/v) with fresh culture medium and organic acids were found to reduce metal toxicity. Among the various organic acids studied, humic acid was the most effective in regulating metal toxicity, apparently due to its multiple binding sites for metal cations. This study demonstrated that environmental factors, such as pH, salinity, extracellular metabolites, and organic acids, can mediate the toxicity of heavy metals to A. doliolum in a laboratory microcosm.