We investigated postnatal development of captive juvenile European hares (Lepus europaeus) with respect to growth, food intake and thermoregulation. Leverets increased their body weight on average 8.5-fold within 35 days, with a peak in relative growth rate on day 8. Although leverets started to take up solid food in the 2nd week of lactation, milk was the main energy source. Leverets of larger litters (two or three young) had less energy supply via milk than those of smaller litters (one young), and digested more energy in form of solid food. Weaning weight was positively correlated with body weight at birth and decreased with increasing litter size. Metabolic rates were largely determined by ambient temperature (Ta) and the increasing body weights. Leverets were able to maintain normothermic body temperatures from the Ist day of life during cold exposure down to Ta -8 degrees C. In their 1st week of life leverets showed reduced rates of heat loss in the cold, possibly by using peripheral vasoconstriction. Our results suggest that the precocial development of leverets is characterised by high maintenance costs due to rapid growth and thermoregulation, which may lead to negative energy balances when food supply is inadequate and T(a)s are low. We hypothesise that these physiological characteristics have played an important role in the recent decline of European hare populations.