Monogamously-mated female albino mice were injected with 0·02–0·50 μg of oestradiol benzoate twice weekly for from 1 to 4 months, and then rested until 6 months had elapsed from the start of the test. All were then injected with 0·20 μg of oestradiol benzoate twice weekly for 2 months. Various control groups were also maintained. The test was then continued for a total period of 12 months, and the fertility of all groups recorded throughout.