The levels of hepatic microsomal cytochrome P-450 in preimplantation rats were determined 1) The activities of the hepatic microsomal cytochrome P-450 associated with the events leading to implantation were gradually increased in preimplantation rats. 2) The level of the hepatic microsomal cytochrome P-450 was peak, 2.65 nmoles/mg protein, on day 4 in normal preimplantation rat. 3) The activity of the hepatic microsomal cytochrome P-450 was decreased in ovariectomized rat than normal rat. 4) The hepatic microsomal cytochrome P-450 level in ovariectomized rat was peak, 2.39 nmoles/mg protein, on day 5. 5) We observed the fact thar the activity of microsomal P-450 was delayed in ovariectomized rat. 6) We confirmed the fact that the hepatic microsomal cytochrome P-450 level was affected by steroid hormone.