Abstract Five patients with acute hepatic failure in coma or stupor were given drugs affecting neurotransmission. Levodopa (L-dopa) produced in three patients a striking though temporary improvement in the level of consciousness together with striking improvement in the previously abnormal electroencephalographic (E.E.G.) recordings. Neither 5-hydroxytryptophan (one patient) nor adrenaline, noradrenaline, atropine, and edrophonium (one patient) affected consciousness or E.E.G. dominant frequency. It is suggested that a defect of dopaminergic neurotransmission is present in hepatic coma.