Cross-fostering in canids, with captive-bred pups introduced into endangered wild populations, might aid conservation efforts by increasing genetic diversity and lowering the risk of inbreeding depression. The gray wolf (Canis lupus lupus) population in Scandinavia suffers from severe inbreeding due to a narrow genetic base and geographical isolation. This study aimed at evaluating the method to cross-foster wolf pups from zoo-born to zoo-born litters. The following was assessed: female initial acceptance of foster pups, growth rate in relation to age difference between foster pups and pups in recipient litters and survival over the first 33 weeks. The study included four litters added by two foster pups in each. The age differences between the foster pups and the recipient litters were 2-8 days. After augmentation, all four females accepted the foster pups, demonstrated by her moving the entire litter to a new den site. Growth rate was dependent on the age difference of the pups in the foster litters, with a considerably slower growth rate in the 8 days younger pups. However, these pups later appeared to be at no disadvantage. Foster pups had a higher survival rate than females' pups, however, the causes of death were probably not kin or non-kin related. The results indicate that cross-fostering works in gray wolves and that this might be a plausible way to increase genetic variation in the wild population.