RESUMEN
Exposure of Anabaena oryzae and Nostoc muscorum cultures to different rates of some common pesticides [dimethoate, dursban, Karathane and trifluralin] resulted generally in pronounced disturbances in nitrogen metabolism, stimulated the process of nitrogen fixation and nitrate reductase activities in both cyanobacteria. Elevation in total fixed nitrogen by either organism was always consistent with respective rise in the amounts of total nitrogen released to the media and total nitrogen contents of both species. Nitrogenase activity in Nostoc muscorum and Anabaena oryzae tended to floculate following pesticides application, depending on the type of the pesticide, its concentration and also incubation period. The increase in potency of nitrogen fixation of both organisms was not accompanied, in some treatments by respective rise in nitrogenase activity. A possible explanation relevant to such contradiction may lay on the interaction of several physical, chemical and biological factors, which can influence the metabolism of the pesticide, altering respiratory and enzymatic potentialities