Approximately 87 per cent of the records fell in the twenty-day interval, May 11 to 30, which is regarded as the main breeding season. Presumably males continue to be at the peak of breeding condition and continue to search for females after the latter have become unreceptive, partly explaining the scattering of records through most of June. Several of the females found in traps with males in May had abundant active sperm in their cloacae and oviducts and probably had been inseminated within a few hours of the time they were checked. Others lacked sperm, but the cramped quarters inside the traps may have effectively prevented the consummation of courtship, especially when two males were confined with the same female. None of the females trapped with males in October was found to be inseminated, and it seems doubtful whether copulation ever occurs at that time of year, although males have motile sperm and seem to be in breeding condition then. _Cycle of the Male_ Cloacal smears indicate that males mature sexually and first produce sperm in August and September when they are a little more than a year old. Insofar as could be determined, there was no sexual activity at this time of year, and actual breeding of the adolescent racers was postponed until the following May. By this time at an average age of 20 months, the snakes had made further growth. [Illustration: Fig. 16. Catches, in semi-monthly periods, of racers in their summer habitats, at Harvey County Park (upper) and at Reservation and Rockefeller Tract (lower). Intensive activity in spring (the breeding season), tapering off rapidly as the season advances, is well shown by the larger sample, but in the years of trapping at Harvey County Park operations usually were not fully underway until the latter part of May.] Mr. Dwight R. Platt studied the changes in the male reproductive organs during the annual cycle at Harvey County. In racers recently emerged from hibernation he found the seminiferous tubules filled with Sertoli syncytium, but containing few germ cells. Spermatogonia proliferate in May and June. During the first half of July primary spermatocytes are the dominant cells in the seminiferous tubules. By early August spermatids are dominant and the first free spermatozoa are present. In late October spermiogenesis is essentially complete and the tubules are relatively empty before the snakes hibernate. During the season of activity the seminiferous tubules increase to approximately double their minimum diameter, reaching the maximum in August. Cyclic changes in size and secretory activity of the ductus deferens, ductus epididymis, and sexual segment of the renal tubules occur, with maximum size and secretory activity coinciding with the time of movement and storage of the spermatozoa. The latter are stored in both ductus deferens and ductus epididymis. Despite the short breeding season, a male racer has active sperm at all seasons. _Eggs_