Tyrosine metabolism via decarboxylation to tyramine was evaluated in dogs with functional end-to-side portacaval shunt. It was found that the endogenous plasma levels of both tyrosine and tyramine increased steadily after the construction of the shunt. These elevations became more pronounced when the dogs manifested symptoms of hepatic encephalopathy. In encephalopathic dogs, average endogenous plasma tyrosine and tyramine concentrations were 110.1 mumoles per liter and 7.6 ng per ml as compared to 55.4 and 1.2 in control dogs, respectively. The pattern of plasma concentrations of tyrosine and tyramine after an oral dose of L-tyrosine (50 mg per kg) was also investigated in control and shunted dogs. There was a progressive rise in peak levels of tyramine (to about 50-fold increase, at 6 weeks) after the construction of the shunt, as compared to levels obtained in pre- and at 1 and 4 weeks postoperatively (70.6 versus 1.20, 3.9, and 8.11 ng per ml). Similar observations were made with levels of plasma tyrosine. Six weeks after portacaval shunt, mean peak levels of plasma tyrosine, achieved at 5 hr after dose administration, were 450 as compared to 85 mumoles per liter obtained in preshunted dogs. These studies demonstrated a correlation between abnormalities in tyrosine metabolism and postshunt hepatic encephalopathy.