Chlamydomonas reinhardii cells, growing photoautotrophically under air, excreted to the culture medium much higher amounts of NO(2) (-) and NH(4) (+) under blue than under red light. Under similar conditions, but with NO(2) (-) as the only nitrogen source, the cells consumed NO(2) (-) and excreted NH(4) (+) at similar rates under blue and red light. In the presence of NO(3) (-) and air with 2% CO(2) (v/v), no excretion of NO(2) (-) and NH(4) (+) occurred and, moreover, if the bubbling air of the cells that were currently excreting NO(2) (-) and NH(4) (+) was enriched with 2% CO(2) (v/v), the previously excreted reduced nitrogen ions were rapidly reassimilated. The levels of total nitrate reductase and active nitrate reductase increased several times in the blue-light-irradiated cells growing on NO(3) (-) under air. When tungstate replaced molybdate in the medium (conditions that do not allow the formation of functional nitrate reductase), blue light activated most of the preformed inactive enzyme of the cells. Furthermore, nitrate reductase extracted from the cells in its inactive form was readily activated in vitro by blue light. It appears that under high irradiance (90 w m(-2)) and low CO(2) tensions, cells growing on NO(3) (-) or NO(2) (-) may not have sufficient carbon skeletons to incorporate all the photogenerated NH(4) (+). Because these cells should have high levels of reducing power, they might use NO(3) (-) or, in its absence, NO(2) (-) as terminal electron acceptors. The excretion of the products of NO(2) (-) and NH(4) (+) to the medium may provide a mechanism to control reductant level in the cells. Blue light is suggested as an important regulatory factor of this photorespiratory consumption of NO(3) (-) and possibly of the whole nitrogen metabolism in green algae.