Viviparity has arisen from oviparity on more than 100 independent occasions in squamate reptiles, providing an unique opportunity to investigate the ways in which endocrine control of gestation length has been modified by natural selection during this major transition in reproductive modes. Intuitively, the evolution of viviparity might be expected to involve an increasingly important role for the steroid hormone progesterone, rather than estradiol. Unfortunately, published data on this topic in snakes are scarce and often contradictory. Females of the viviparous snake Vipera aspis reproduce with a lower than annual frequency, providing the opportunity to examine steroid profiles simultaneously in vitellogenic, pregnant, and postparturient versus nonreproductive females. From 1990 to 1994, more than 500 blood samples were collected from more than 100 females. Progesterone, estradiol-17β, and several plasma metabolites were assayed by radioimmunoassay and spectrophotometry. In contrast to earlier studies, we found significant differences between plasma progesterone levels in reproducing (10.5±9.1 ng ml-1, N = 168) and nonreproducing (5.1 ± 4.2 ng ml-1, N = 121) females. Estradiol reached high levels during vitellogenesis (4.8 ± 4.0 ng ml-1,N = 16), and progesterone levels increased during gestation (from 3.7 ng ml-1 before gestation to 18.7 ng ml-1 at midgestation). However, experimental elevation of plasma progesterone levels with implants (up to 44.4 ng ml-1) did not modify plasma metabolite levels or delay parturition.