Synthesis rates of albumin and fibrinogen were determined in six patients with acute virus hepatitis and in nine control patients using the 14C carbonate method of McFarlane. Five patients were restudied several weeks after complete recovery. The following results were obtained. (1) The mean albumin and fibrinogen synthesis rates in the control group were 238.8 and 23.5 mg/kg/day, respectively. (2) In two patients with mild courses of hepatitis and in one patient studied during the early phase of a rapid and fatal course the albumin synthesis rate during the acute disease did not differ from controls whereas fibrinogen synthesis rate was slightly increased. (3) Three patients with severe hepatitis showed a definite decrease in albumin and fibrinogen synthesis rates, the lowest value being 76 mg/kg/day for albumin and 6.3 mg/kg/day for fibrinogen. (4) The decrease in plasma protein synthesis correlated neither with serum transaminase or bilirubin levels nor with plasma albumin or fibrinogen concentrations during the acute phase. (5) The two patients with the lowest albumin and fibrinogen synthesis rates also showed a definite decrease of the prothrombin index during the acute phase. Both patients presented very prolonged courses of the disease. (6) In all patients studied twice, plasma protein synthesis rates were normal after recovery.