During recent years increasing attention has been paid to the action of diphenylhydantoin on the pain paroxysms in trigeminal neuralgia (tic douloureux). Many authors are of the opinion that the drug is of real value in this condition. The effects of diphenylhydantoin on synaptic transmission in the spinal cord of the cat has been throughly studied by Esplin,3 who found a slight weakening of the monosynaptic reflexes and a more pronounced reduction of the polysynaptic reflexes in the lumbar section of the spinal cord. A most striking effect was found on the post-tetanic potentiation (PTP). After intravenous injection of lidocaine (Xylocain) Bernhard and Bohm1 observed a slight reduction (to 65%-75%) of monosynaptic and polysynaptic reflexes in the lumbar region of the spinal cord in cats (curarized and under light pentobarbital [Nembutal] anesthesia). Kugelberg and Lindblom5 found an increase in the summation time and decrease in the duration