Evidence that indoleacetic acid (IAA) conjugates are metabolized via enzyme-catalyzed hydrolysis to free IAA and that their biological activities are related to the rates at which they are hydrolyzed by the tissue is presented. These conclusions are based on the following observations. Slow but continuous decarboxylation of the IAA moiety of IAA-l-alanine and IAA-glycine occurs when these conjugates are applied to pea (Pisum sativum L. cv. Alaska) stem segments. Inasmuch as IAA conjugates are protected from peroxidase-catalyzed oxidative decarboxylation, the conjugates are probably hydrolyzed and the freed IAA then further metabolized. Free IAA and IAA-l-alanine are converted, by pea stem tissue, into the same metabolites. The metabolism is enzymic, since conjugates of IAA with the d-isomers of the amino acids are inactive. Ethylene production induced by IAA-l-alanine and by IAA-glycine is correlated with their hydrolysis, as indicated by their decarboxylation and with the appearance or nonappearance of IAA metabolites in the tissues.