Recorded dates of mating for the species are all in spring, but indicate a span of many weeks for the breeding season, and this spread results in part from geographical differences. Published records are as follows:
Subspecies constrictor
May 12, 1930, in Ohio (Conant, 1938:55)
Subspecies priapus
May 8, 1921, in Georgia (Wright and Wright, 1957:135)
May 9, 1921, in Georgia (Wright and Wright, 1957:135)
Subspecies flaviventris
May 3, 1931 (two pairs) in Missouri (Boyer and Heinze, 1934:195)
April 18, 1936, in Missouri (Anderson, 1942:210)
May 12, 1928, in Kansas (Gloyd, 1928:123)
Subspecies mormon
June 10, 1927, in Utah (Cottam, 1937:229)
July 7, 1938, in California (Cunningham, 1959:17)
In the course of my live-trapping, I occasionally found more than one racer in a trap. As might be expected from the low yield per trap, such double or multiple captures were relatively rare. Chance, and unusually strategic placement of certain traps were doubtless contributing factors. May and October, being the most productive months for trapping, yielded a high proportion of these combined captures. Some involved an adult and an immature snake, or two adults of the same sex. Eliminating all these, there remain 44 heterosexual captures of adults. These latter captures are significantly concentrated in their seasonal distribution and indicate a spring breeding season; 34 were in May, six were in June and four were in October. Eight of the May records and one June record each involved a trio of snakes—two males and a female in every instance. Distribution of the spring records, grouped in five-day intervals, was as follows:
| May 11 to May 15: | 14 |
| May 16 to May 20: | 5 |
| May 21 to May 25: | 8 |
| May 26 to May 30: | 13 |
| May 31 to June 4: | 1 |
| June 5 to June 9: | 0 |
| June 10 to June 14: | 2 |
| June 15 to June 19: | 2 |
| June 20 to June 24: | 0 |
| June 25 to June 30: | 1 |
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.
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.
