Nitrate reductase activity was induced in Chlamydomonas reinhardi following addition of nitrate. Enzymic activity was assayed in cell-free supernatants and in whole cells whose permeability had been increased by freezing. Nitrate reductase activity in cells decreased rapidly when CO2-fixation was prevented by (a) darkening cultures, (b) aerating cultures with CO2-free air, or (c) addition of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). A smaller loss of nitrate reductase activity from darkened cells occurred when (a) acetate-adapted cells were supplied with acetate, or (b) cells were allowed to accumulate carbon reserves by nitrogen starvation before darkening. It was concluded that maintenance of nitrate reductase activity was dependent upon the availability of a suitable carbon source.