Blood ammonia levels for the femoral artery and jugular bulb were detd. on 3 patients with hepatic coma of varying degree, 3 non-hepatic patients in coma, and 3 non-comatose, non-hepatic patients. For the 6 non-hepatic patients the femoral arterial levels were in the range 0.33-0.92 gamma NH3N/ml. whole blood, the jugular bulb levels were 0.31-0.97 gamma/ml. and the arterio-venous difference, -0.08-0.02 gamma/ml. (normal venous range 0.3-1). The corresponding values for the patients in hepatic coma were 2.81-8.44, 2.32-7.12, and +0.31 - +1.32. In view of the marked uptake of ammonia by brain the hypothesis is presented that a reductive amination of ketoglutarate might be taking place in brain in coma, resulting in removal of significant amts. of ketoglutarate from the Krebs cycle, thereby preventing regeneration of the oxidative cycle.