A multi-year radio-telemetric study of the copperhead Agkistrodon contortrix (Serpentes: Viperidae) was conducted at the north-eastern extreme of its range to determine the relationship of plasma sex steroids of males to the mating season. Blood samples were collected in situ approximately every 2 weeks (repeat-test group) from radio-telemetered males during the 7-month active season (April–October) from 2001 to 2003 and assayed for concentrations of testosterone (T) and progesterone (P4). Blood samples were also obtained from a large number of incidental males (single-test group) for the analysis of seasonal levels of T and P4. The profiles of T and P4 showed a peak in August–September that corresponded to the single mating season (late July to late September). Both T and P4 had similar seasonal profiles, but absolute levels of these steroids were significantly different, with concentrations of T four- to fivefold greater. The mating season of the population we investigated differs from other (e.g. southern) populations, which show two mating seasons (late summer/early fall and spring) before the period of ovulation in mid- to late spring. When a mating season is absent in spring, inseminated females are obligated to store sperm over winter until ovulation in the spring. In studies of A. contortrix that document two mating seasons, peak levels of T in males are coincident with both of these periods. In contrast, we found that peak levels of T and P4 in males coincided with the occurrence of the single mating season, and levels were basal in spring. These results further support the idea that plasma sex steroids influence the expression of sexual behaviour in A. contortrix and other snakes. Nonetheless, confirmation of this view awaits experimental studies.