One major goal of physiological ecology is to seek links between life history traits. Identification of a body condition threshold for breeding (e.g. critical level of body reserves) provides a link between the physiological process involved in storage of body reserves and the ability to reproduce. One hundred and twenty-nine freeranging adult female Vipera aspis, a viviparous snake, were marked with electronic identification tags and/or by scale clipping, weighed, and measured at the onset of vitellogenesis, and immediately released in the field in western central France (47Β°03'N; 02Β°00'W). The 129 snakes were recaptured 2-6 months later between ovulation and parturition, and individual reproductive status was then determined. Eighty-four females (65%) captured at the onset of vitellogenesis became vitellogenic, 45 did not. There was no difference in mean body length between reproductive and non-reproductive females. Initial body condition influenced reproductive outcome: we found a precise threshold in body condition necessary for the induction of vitellogenesis. Almost all females (88%) with a body condition greater than the observed threshold became vitellogenic, 12% did not, and no female with a body condition under the threshold became vitellogenic. Body reserves were estimated in the 129 living females using data gathered on 69 autopsied females. Females which became vitellogenic had large body reserves, but females which did not were not particularly emaciated (whilst postparturiant females had few body reserves remaining). This precise condition threshold for breeding is discussed in terms of the reproductive ecology of this species.