Adrenalectomy before pregnancy in rats caused the persistence of high blood levels of prolactin (PRL) throughout the ensuing postpartum lactation. The persistence of hyperprolactinaemia was without effect on the (delayed) timing of blastocyst implantation during lactation. The findings indicate that ovarian cycles and pregnancy may continue normally despite the absence of adrenal hormones. They reveal that the enhanced release of pituitary PRL in response to suckling is not dependent on the removal of the adrenals during early lactation. The normal delay of blastocyst implantation through suckling, in the presence of abnormally high concentrations of PRL in blood, throws doubt on the supposed critical role of PRL in determining the length of the period of delay of implantation during lactation.