The objective of our work was to provide allometric relationships between different body measurements and age of young leverets of European hare Lepus europaeus (less than two months old) for use in live-trapping studies. Five morphometric variables were measured to predict the age of leverets: body mass, skull length, skull width, hind foot length and ear length. The hares came from two rearing centres near Paris (France) and were handled in 1995 and 1996. Measurement repeatability of each variable was compared in a pilot study (N = 51 leverets) and the effect of rearing centre on the growth curve was tested for the first 30 days after birth. We used a complementary sample (N = 168 leverets) to establish a model of age determination using I-splines regression. The skull length was the best candidate variable based on measurement repeatability, comparability between the two rearing centres and goodness of fit. I-spline modelling has minimal underlying statistical assumptions. In addition, it provides easy-to-use and powerful plots of the relation between age and measurements, including confidence limits estimated by a bootstrap procedure.