Little is known concerning the succession of events which occur soon after the introduction of virulent Treponema pallidum into a susceptible or a resistant host. It is known that with the experimental methods commonly employed, treponemes rapidly migrate or are carried beyond the site of inoculation to invade the regional lymph nodes and the blood stream. It is also known that after about 5 days minimal histological changes can be recognized in susceptible animals, although microscopically detectable lesions do not often occur before the 10th to 14th day. The present study is an attempt to determine what happens to spirochetes during the first few days following introduction into a susceptible or a resistant host. Groups of rabbits having testes of approximately the same size were inoculated with an emulsion prepared from testicular syphilomas, Nichols strain, and diluted so that the desired number of spirochetes was contained in a volume of 0.2 cc. Care was taken to place the inoculum into the middle of the testis. At various intervals after inoculation testes were removed and cut into six equal transverse sections. A loopful of serum-saline was pressed into the surface of each segment and the tissue-rich fluid collected on a coverslip. Using a darkfield microscope 50 high power fields of each preparation were examined, and the total number of spirochetes counted in all six preparations was taken as an index of the spirochete population of the whole testes. Immune rabbits were obtained by intracutaneous inoculation of the back with the Nichols strain of T. pallidum. Seventy to eighty days after infection the animals were treated with a curative dose of penicillin in order to eliminate any spirochetes present in the testis prior to intratesticular introduction of the challenge inoculum.