During nitrogen assimilation by Chlorella a change in the source of this element produced a substantial effect on the activity of NADP-glutamate dehydrogenase and a smaller effect on the activity of alanine dehydrogenase. The activity of NAD-glutamate dehydrogenase was almost unaffected. NADP-glutamate dehydrogenase activity drastically increased upon cultivation on ammonium salts and decreased upon cultivation on nitrate. These changes resulted from an enhancement or reduction of the synthesis of enzymic protein. Glutamine, alanine, and α-ketoglutarate exerted no effect on the activity of the tested enzymes.