Delayed implantation, after ovariectomy on Day 3 of gestation, was induced in mice by subcutaneous injections of 6α-methyl-17-acetoxyprogesterone on Days 3 and 8 of gestation and of oestradiol benzoate on Day 8. 95% of mice had implantations on Day 12. The mean number of implantations per mouse, 11·3, was not significantly different from the mean in normal controls. Delayed implantation sites were similar in size to those of normal mice when allowance was made for the delay, while the incidence of visibly pathological sites was low. Variations of the technique were less effective.