In an effort to determine the sum of the factors influencing the plasma and brain concentrations of aromatic amino acids in rats with liver disease, rats with end-to-side portacaval shunts and their sham-operated controls were placed on various oral and intravenous diets by the techniques of parenteral nutrition. In sham-operated animals with normal hepatic function, the source of amino acids and their composition appeared to play little role in the plasma and brain aromatic amino acids and a derivative of brain tyrosine, octopamine. After end-to-side portacaval shunt, however, plasma and brain concentrations of aromatic amino acids were responsive to exogenous intake in the presence of hepatic impairment. In a group of rats with end-to-side portacaval shunt and presumed hepatic impairment, graded increments of protein equivalent were given with isocaloric hypertonic dextrose by the techniques of parenteral nutrition. When the animals achieved positive nitrogen balance, there were decreased levels in plasma and brain of the aromatic amino acids, phenylalanine and tyrosine, and a derivative of tyrosine, octopamine. A clear statistical relationship between plasma and brain tryptophan and nitrogen balance was not observed. The results suggest that in animals with hepatic impairment secondary to end-to-side portacaval shunt both exogenous intake and the level of catabolism (nitrogen balance) influence plasma and brain aromatic amino acids.