We quantified embryonic growth in 10 species of the viviparous thamnophiine snakes and three oviparous colubrid snakes. We fit data on mass, total length, and snout-vent length to linear and exponential growth models; all three size measures (particularly the lengths) fit the linear model better. For all three measures, there were significant differences among species in the absolute growth rates obtained from the linear model; these correlated with differences in neonate size. For the relative growth rates obtained from the exponential model, significant differences among species were seen only for mass; these were only moderately correlated with neonate size. Gestation periods, estimated from absolute growth rates and neonate size, generally increased with size. Comparisons with outgroup species suggest that having large neonates is the ancestral condition for the thamnophiines and that small neonate size evolved at least twice within the clade. Size reduction was achieved through lower embryonic growth rates and shortened gestation periods. Comparisons also indicate that differences in growth rates for total length and snout-vent length relative to each other contribute to differences in body proportions among species.