AbstractFree and bound amino acids in the mycorrhizal and nonmycorrhizal root systems ofPinus nigraArnold andCorylus avellnaL. grown under semi‐natural conditions were analyzed through automatic amino acid analyzers.Tuber brumaleVitt. andT. melanosporumVitt. were the respective fungal symbionts. Arginine and citrulline were found to accumulate in large quantities in the free pool of the uninoculated P. nigra and C. avellana root systems respectively. In the mycorrhizal root systems these substances decreased in their levels with a parallel increase in the concentrations of glutamine and asparagine. Implications of these changes are discussed with reference to ectomycorrhizae. In general the majority of the identified free amino acids were found in larger concentrations in both the types of mycorrhizal root systems. Results obtained suggested that this may not be due to proteolysis but due to increased biosynthesis. Possible interrelationships in ectomycorrhizal root systems with reference to nitrogen metabolism are presented in the form of a schem