The effects of l-thyroxine and of surgical thyroidectomy upon the survival of blastocysts have been studied in 200 albino rats. These rats were ovariectomized on the 3rd day of pregnancy, and maintained on progesterone in order to delay implantation. Delay of implantation was confirmed by laparotomy on the 8th day of pregnancy. Subsequent implantation was accomplished by giving 1 \ g=m\ g oestrone daily from the 9th day of pregnancy. At autopsy on the 14th day of pregnancy hyperthyroid rats and hypothyroid rats which had been maintained on daily injections of 2\m=.\0 mg progesterone did not differ from their respective control groups in the number of surviving blastocysts. However, hyperthyroid rats which had been maintained on daily injections of 0\m=.\4mg progesterone possessed more implantation sites than controls. Similarly, the hypothyroid rats maintained on daily injections of 0\m=.\3 mg progesterone had fewer implantation sites than controls. The experiments suggest that the level of hyperthyroidism tested is beneficial to the maintenance of implantation of delayed blastocysts when low amounts of progesterone are available, while hypothyroidism tends to be detrimental to this process during low progesterone availability.

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