An experiment, designed to determine the extent of the maternal influence on size in sheep, in which groups of Border Leicester and Welsh Mountain ewes were bred to rams of both breeds, has shown that the nature of the maternal environment had a relatively small effect on the size of lambs at birth and on their subsequent growth. The maternal influence on a late-maturing character (liveweight) was larger than on an early-maturing character (’ cannon-bone’ length), but the effect of heterosis on the late-maturing character was greater still and tended to overshadow the maternal effect.