Eighty of 640 4-day and 5-day ova transferred into the uterine horns of two strains of rats on Day 4 of pseudo-pregnancy developed into foetuses that were weighed on Day 18 of pregnancy. The foetuses developing from 5-day ova weighed significantly more than those resulting from 4-day ova. Analysis of three other factors contributing to foetal weight showed that none of these differed significantly from zero. One additional day of development before implantation gives ova a selective advantage over normally developed controls in terms of foetal weight at term. Further experiments are in progress to study this effect in more detail, and in other species.