Growth during an animal's early ontogeny can have important consequences for its health, reproduction and survival during later life. We studied pre-weaning growth rates of two species of small altricial mammals, and assessed the impact and compared the importance of two main aspects of the animals' early environment: maternal characteristics and the number of litter siblings (i.e. litter size). Study animals were wild-type European rabbits living in a field enclosure and laboratory rats kept under standardized housing conditions. Growth of rabbit pups was best explained (to 47%) by the combined effects of litter size, maternal age and whether females had given birth to another litter shortly before. Similarly, growth in rat pups was best explained (to 75%) by the combined effects of litter size, maternal body mass and whether mothers had previously given birth. In both species, litter size correlated negatively with pup growth. In rabbits, growth was greatest in litters of middle-aged females. In rats, pup growth correlated positively with maternal mass. Pups of both species showed higher growth rates when the mother had not given birth to a previous litter. Despite major differences in maternal behaviour and study conditions between the two species, the findings point to a similar ranking in the importance of the different parameters tested: Litter size, i.e. sibling number, was the most important factor, followed by maternal mass or age, and then by mothers' history of previous reproductive activity. We therefore think it likely that these findings will be valid for other mammalian species giving birth to large litters of altricial young.