An estimate of the total energy intake of litters until weaning, including both energy obtained in-utero and that derived post-natally from milk consumed, is often required in experiments on growth efficiency. Frequently it is, however, impossible to measure this energy directly. Consequently, a method was developed using litter mass to estimate the cumulative energy intake by rats to weaning. An equation was also developed to divide this cumulative energy intake of a litter between the individuals of the litter.