Expression of nitrite uptake and nitrite reductase activities has been studied in Chlamydomonas reinhardtii under different nutritional conditions. Both activities were expressed at a low level in derepressed cells (with no nitrogen source) and at a high level in induced cells (with nitrate or nitrite). Nitrate was required for both activities to be maximally expressed. Ammonium-grown cells did not show nitrite uptake capability and had a basal nitrite reductase activity. Nitrite uptake but not nitrite reductase levels decreased very significantly in nitrate-induced cells subject to cycloheximide treatment, which suggests that protein(s) involved in the uptake are under a rapid turnover. Nitrite uptake expression was strongly inhibited by the presence of the glutamine synthetase inhibitor L-methionine-D,L-sulfoximine under either derepression or induction conditions, whereas that of nitrite reductase was not affected under the same conditions. Our results indicate that nitrite uptake expression is regulated primarily by ammonium, and that of nitrite reductase by both ammonium and ammonium derivative(s).