Rats and mice ovulate and are receptive to males on the 1st or 2nd night following parturition. An animal may thus be pregnant while it is lactating, but implantation is often delayed (Lataste, 1891). In general, it appears that the length of the delay in implantation is correlated with the number of suckling young (Enzmann, Saphir & Pincus, 1932).
The data concerning delayed implantation in lactating rats and mice have been collected from relatively small numbers of experimental animals. The Charles River Breeding Laboratories, a commercial producer of rats and mice for scientific research, have made available to us breeding records of a larger population from which it is possible to estimate the frequency of delayed implantation and the length of the delay.
One pair of Charles River CD* rats (derived from Sprague-Dawley) or Charles