The paper deals with a sample of 185 pregnant rabbits obtained in Caernarvonshire between February and July 1941 inclusive, during the height of the breeding season. The intra-uterine mortality was estimated by comparison of the number of corpora lutea in the ovaries with the number of healthy embryos in the uteri. By grouping the data according to the stage of development of the embryos, it is shown that between 42 and 56% of all litters are lost before the 20th day of gestation. This enormous mortality occurs chiefly on the 11th and 12th days of gestation, after implantation and when the placenta is already well established. Analysis of the mortality according to the initial size of litter shows that the proportion of litters showing loss increases with the initial size of the litter in the form of a geometrical progression. In almost every litter-size class a relatively large number of litters were observed in which all the embryos were reabsorbing. The mortality, whether represented by the percentage of animals showing loss or by the percentage of ova lost, also varies according to (a) whether the animals are, or are not, lactating, (b) the cleaned body weight of the mother, and (c) the month. These variations are, however, fully accounted for by variations in the frequencies of the initial size of litter.