The Chlamydomonas reinhardtii Nar1;1 gene encodes a membrane protein involved in nitrite transport to the chloroplast and is apparently dispensable for nitrate/nitrite-dependent growth in some conditions (Rexach, Fernández & Galván, Plant Cell 12, 1441-1454, 2000). In the present study the question of when NAR1;1 is required and how it improves nitrate utilization for growth was investigated. Comparative studies were performed with strains Nar1;1+ and Nar1;1- in nitrate media under three experimental conditions. Under continuous light and high CO2 (5%), strains containing or lacking Nar1;1 showed a very similar nitrate-dependent growth. Under continuous light and low CO2 (air), Nar1;1 allowed a slightly higher growth in nitrate. However, under light/dark cycles and low CO2, the strain having Nar1;1 showed a higher nitrate-dependent growth and doubled the amount of protein and chlorophyll than the Nar1;1- strain. The activities for nitrate uptake, nitrite and ammonium excretions, nitrate and nitrite reductases, glutamine synthetase (GS), as well as the transcript levels of Nar1;1, Nrt2;1, Nar2, Nrt2;3 and GS1 and GS2 genes were determined under this third condition by trying to understand how NAR1;1 improves nitrate use efficiency. Nitrite and ammonium excretion, which was higher in the strain Nar1;1- than in Nar1;1+ results in an important deregulation in the gene expression of nitrate transporters and cytosolic GS1. These data lead to the proposal that nitrite transport to the chloroplast mediated by NAR1;1 allows a balanced expression of enzymes and transporters for nitrate assimilation, thus optimizing nitrate utilization under the natural condition of light/dark cycles and air.