In multiparous mammals there is a definite relation between the litter size N and the total weight of the litter W. Reasons are given showing that this relationship is independent of the mechanism of parturition. For various forms W = NK + const. Hence the average increment of W due to unit increase of N is directly proportional to W, inversely to N. This signifies proportionality between N and nutritive drain upon the mother, as well as equipartition among the members of the litter. K is non-specific, and is therefore regarded as a partition index.