Summary. Reproductive performance before parturition has been investigated in laboratory mice of two breeding systems, each of two highly inbred strains and their F1 progeny. More corpora lutea of preg- nancy were found in the BUB and I/Fn strains than in the C57BL/10J and A/Fn strains respectively, or in the F1s derived from the pairs contrasted. BUB and I/Fn females had more pre-implantation and fewer post- implantation losses than females of the C57BL/10J and A/Fn strains, while F 1 females lost proportionately fewest potential young both before and after implantation. The number of conceptuses lost at different times during pregnancy differed proportionately in the populations examined and the BUB females differed from all others in not having a marked rise in embryonic mortality at about the 10th day of gestation. The significance of this departure from the norm is discussed.