Methods are described for the assay of nitrate reductase in frozen/thawed cells of Ankistrodesmus braunii and Chlamy domonas reinhardi. Because Ankistrodesmus nitrate reductase can use NADPH as an electron-donor and because the cells of Ankistrodesmus contain a high concentration of glucose-6-phosphate, it is suggested that, in frozen-thawed cells, nitrate reduction is coupled, through NADPH, to the oxidation of glucose-6-phosphate. With Chla-mydomonas cells either exogenous NADH or reduced FMN can act as reductant. The permeability of Chlamy domonas cells can also be increased by glycerol treatment and this makes assay of nitrate reductase possible without freezing and thawing.