The activities of glutamic acid decarboxylase (GDC), glutamic–oxaloacetic transaminase (GOT), and glutamic acid dehydrogenase (GDH) were determined in cell-free extracts of healthy and rust-infected sunflower cotyledons and uredospores. Uredospore extracts contained an active DPN-linked GDH, but little or no GDC or GOT activity. The activity of GDC was lower and that of GOT higher in extracts of rust-infected tissue as compared to healthy tissue due to a redistribution of their coenzyme, pyridoxal phosphate. The activity of GDH was higher in 'rusted' extracts due to the presence of the similar fungal enzyme. It is suggested that this contribution of GDH by the fungus to the host–parasite complex alters the normal coenzyme pattern of other enzymes metabolizing glutamic acid.