Animals fed a protein free diet (PFD) without steroid therapy failed to maintain pregnancy. Injection of 0.5 μg oestrone and 6 mg progesterone daily between days 3 and 20 or days 3 and 12 of pregnancy allowed 100% pregnancy maintenance. Restriction of the steroid injection regime to between days 7 and 9 of gestation reduced pregnancy maintenance to 36%. Prolactin (50 IU subcutaneously twice daily) allowed 78% pregnancy maintenance. The evidence suggests that protein deprivation acts rapidly during early pregnancy to reduce hypophysial secretion of the hormones necessary for pregnancy maintenance, specifically prolactin.