When injected into the common carotid artery the convulsive dose of aminopyrine in dogs was 2.5 mg/kg, while into the femoral artery or vein it was about 30 mg/kg. After unilateral ligation of the internal carotid artery the convulsive dose of aminopyrine injected into the ipsilateral common carotid artery increased greatly and even exceeded the intravenous dose. Following unilateral ligation of the external carotid artery a dose of less than 2 mg/kg of aminopyrine induced convulsions, when injected into the common carotid artery of the same side. The convulsive dose of aminopyrine was even lower (0.6 mg/kg) when injected directly into the internal carotid artery (following ligation of this vessel below the site of injection).