Hepatic encephalopathy (H.E.) is associated with and perhaps caused by changes in plasma-aminoacid patterns--decreased branched-chain aminoacids (B.C.A.A.) and increased aromatic aminoacids (A.A.A.). The decreased B.C.A.A. may be in part secondary to hyperinsulinaemia, but the B.C.A.A. are catabolised by both fat and muscle. The increase in A.A.A. may reflect a "catabolic stimulus" reflected in hyperglucagonaemia, particularly in severe hepatic failure and H.E., and a decreased insulin/glucagon ratio. Endogenous protein, lean body-mass, or liver then releases large amounts of A.A. and the A.A.A. cannot be catabolised by the failing liver, and thus accumulate in the circulation. With decreased plasma-B.C.A.A., the molar ratio of B.C.A.A. and A.A.A. decreases allowing the toxic A.A.A. to penetrate the blood-brain barrier in increased amounts and encephalopathy develops. Appropriate therapy for H.E. must include reversal of the "catabolic state" by providing sufficient B.C.A.A. and calories to decrease the flux of A.A.A. from muscle and liver, and the restoration of the normal molar ratio of B.C.A.A. and A.A.A.