1. 1. Hemisection of the spinal cord, at C-1, or unilateral intraspinal injection of novocaine in acute experiments on cats, leads to an asymmetrical electrocorticogram. 2. 2. The asymmetry is due to preponderant electrocortical synchronization (ECS) on the hemisphere ipsilateral to the spinal intervention. 3. 3. A small contralateral ECS also usually results from unilateral interference at C-1. 4. 4. In some cases, ECS appears to be bilaterally equal, but various tests indicate the greater potentiality of the ipsilateral cortex for synchronization. 5. 5. These findings are considared in relation to similar predominantly, but not exclusively, ipsilateral ECS resulting from lesions and local anesthetization of the mesencephalon. 6. 6. A common mechanism resulting from impairment of the ascending reticular activating system might be responsible for both cord and mesencephalic asymmetries. Anatomical justification for such a possibility is cited.