Batch cultures of Chlorella fusca excreted nitrite into the medium if gassed with air (0.03% CO2), but they did not if supplied with air containing 5% CO2. After a change from high to low CO2 concentration in the gas stream, nitrite excretion started immediately. After an increase in CO2 concentration to 5%, nitrite uptake started within only 30 min. Changes of in-vitro activities of nitrate reductase, nitrite reductase and glutamine synthetase did not correspond to changes of nitrite concentration in the medium and therefore could not explain these observations. A nitrite-binding site, whose activity corresponded with both nitrite excretion and uptake, was detected at the chloroplast envelope. From these data an additional regulatory step in the assimilatory nitrate-reduction sequence is suggested. This includes an envelopeprotein fraction probably regulating the availability of nitrite within the chloroplast.