While the rabbit is an animal well adapted for chemotherapeutic studies in syphilis, nevertheless, when one is concerned with drugs intended for the treatment of dementia paralytica, the usefulness of the rabbit for the testing of the efficiency of such drugs is definitely limited by the fact that the spirochetes of syphilis are ordinarily unable to penetrate into the central nervous system of the rabbit, or to persist there for appreciable periods of time if directly introduced ( Levaditi and his associates1 and Raiziss and Severac2). Even with animals displaying severe generalized lesions, with the infection running a course of several years, in some cases, it is impossible to discover any spirochetes in the brain either by means of a direct microscopic observation or by inoculation of emulsified brain substance into the testes of normal rabbits.2b Only a few cases have been reported in the literature