We wished, in our study, to find the dose of anesthetic agent required, when given subdurally, to produce permanent paralysis of the hind legs and permanent changes in the spinal cord. Since procaine hydrochloride is the safest and most universally employed drug for spinal anesthesia, our experiments were confined to its use. The data here reported deal chiefly with results following subdural injection of huge doses of procaine in the dog. It was found necessary to give very large doses of procaine, in definite concentration, to produce permanent ill effects, as judged by the behavior of the dog and the microscopic examination of the spinal cord. Davis, Haven, Givens and Emmett1 have reported that a dose of procaine suitable for man, when given subdurally to a dog, has resulted in changes in the membranes of the spinal cord and in the spinal cord itself; these changes, however, were not