Plasma free, sulfoconjugated, and glucuronoconjugated CAs were determined radioenzymatically before and after hydrolysis. CA sulfates and glucuronides were hydrolyzed by sulfatase and Ketodase, respectively. Total CA conjugates were deconjugated by acid lyophilization. On the basis of data obtained from systematical studies on these methods, the conjugation patterns of endogenous plasma CA in human and rat have been qualitatively and quantitatively identified. The results show that (1) in human plasma, sulfation is the sole conjugation mode for DA, NE, and E; (2) in rat plasma, sulfation also occurs in DA, NE, and E but the quantities of the sulfates are small, whereas glucuronidation predominates in DA and occurs exclusively in DA but not in NE and E; (3) in both species, conjugation is preferential for DA rather than NE and E. In this study, a new specific method for analysis of glucuronoconjugated CA has been developed.