The leaf-litter fungus Coprinus cinereus maintains a pool of free amino acid in its mycelium. When the organism is grown under conditions of high nitrogen availability with 13.2 mmol.L-1 L-asparagine as the nitrogen source, the primary constituents of this pool are glutamine, alanine, and glutamic acid. Together these 3 amino acids comprise approximately 70% of the pool. Nitrogen deprivation reduces the size of the free amino acid pool by 75%, and neither a high concentration of ammonium nor a protein nitrogen source support a similar pool size as L-asparagine. Nitrogen deprivation also reduces the concentration of glutamine to the pool while increasing glutamate. Concomitant with this shift is a marked increase in mycelial ammonium.