We studied 36 pregnant females and 70 litters of Vipera aspis. Vipers were either recently caught, living in outdoor terrariums (semi-natural conditions), or living in the laboratory (artificial conditions). Following results were obtained: 1. There were significant correlations between the number of eggs and the weight of the litter and between the length and weight of the females. 2. The weight of hatchings does not depend, or only to a small extent, on the length or on the weight of the female, but is negatively correlated with the number of eggs. Hatching weight is generally higher in semi-natural and artificial conditions than in natural conditions. On the other hand, litter size was independent of the rearing conditions. 3. The weight of a litter at birth represents an average of 44 and 47% of the female's weight, respectively in natural and semi-natural conditions, where vipers breed generally once every two years; and only 34% in artificial conditions, where females breed once or twice a year. In all cases, this proportion is independent of the female's weight.