It is now a well established fact that high-level nutrition of the in-lamb ewe during the later stages of pregnancy results in heavy lambs of high vitality at birth and in good milking ewes. On the other hand, when ewes are poorly fed during the same period, or suffer a check, they produce underweight lambs of low vitality and have poor milk yields. While it is agreed that underweight lambs are undesirable, one is often asked whether the larger and sturdier lambs at birth maintain their superiority in later life, a problem on which relatively few results have been published. A study has therefore been made of a large number of birth and weaning weights collected as a normal routine at the Rowett Institute during the past few years.