Two glutamate dehydrogenase isozymes were shown to exist in Chlorella pyrenoidosa (strain 7-11-05) and the cellular level of these isozymes was regulated by the nitrogen source upon which the algae were cultured. One isozyme was induced by ammonium and was specific for the coenzyme, triphosphopyridine nucleotide (TPNH). The isozyme present in cells cultured in nitrate-containing medium has a much higher specificity for the coenzyme, diphosphopyridine nucleotide (DPNH). The TPNH:DPNH activity ratios for the respective isozymes were 33:1 and 1:5 and their respective molecular weights were calculated to be 269,000 and 179,000. The DPNH-specific isozyme was synthesized throughout most of the cell cycle of light-dark synchronized cells. The level of the TPHN-specific isozyme appeared to be negligible in the nitrate-cultured cells. The induction of the TPNH-specific isozyme was dependent on both RNA and protein synthesis and the isozyme was inducible at all times during the cell cycle. The maximum rate of induction or the potential increased during the period of DNA synthesis and the fold increase in potential and in DNA were equivalent. This data supports the continuous availability for transcription of the gene for this isozyme in the eucaryote Chlorella and is consistent with the hypothesis that, under fully induced conditions, the gene dosage of the cell governs the potential. / Ph. D.