In this paper, we present results of our studies on nitrogen metabolism in the green alga Monoraphidium braunii, strain 202-7d, concerning changes in intracellular ammonium, glutamate and glutamine pools (as determined by reversed phase chromatography). We found that blocking of nitrogen assimilation at the GS or GOGAT steps quickly induced changes in intracellular amino acid levels, which were opposite in the case of Gln. Monoraphidium cultures can recover from the effect of such inhibit ors, probably by its metabolization. Darkness or addition of DCMU affected GS regulation in the same way in Monoraphidium, but they provoked distinct effects on intracellular pools of amino acids. The inhibition of GS activity had a similar effect on the levels of primary products of ammonium assimilation (Glu and Gln) regardless of the energetic state of cells.