The conidiation process of Neurospora crassa is characterized by three morphogenetic steps: hyphal adhesion, aerial hyphal formation, and production of conidia. Previous data indicated the occurrence of a hyperoxidant state at the onset of all three morphogenetic steps. Because glutamine synthetase (GS) and the biosynthetic glutamate dehydrogenase [GDH(NADP)] enzymes are susceptible to inactivation by reactive oxygen species, we followed these enzyme activities during conidiation and under different physiological conditions and related them to the hyperoxidant states and morphogenesis. Loss of GS activity occurred prior to all three morphogenetic steps, coinciding with an increase in total protein oxidation. Oxidized GS polypeptides were detected during hyphal adhesion. Loss of GDH(NADP) activity also occurred during hyphal adhesion and before aerial hyphal formation; the enzyme polypeptide and activity decreased in the adhered hyphae to low values and no GDH(NADP) was detected in aerial hyphae. The catabolic GDH [GDH(NAD)] behaved in an opposite manner, increasing its activity during hyphal adhesion and aerial hyphae development. These results are discussed with regard to cell differentiation and the conidiation process in N. crassa.