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.

Cytochrome P-450 is sometimes called the microsomal carbon monoxide-binding pigment. It is present in the microsomal fraction of several animal tissues, and also in the mitochondrial fraction of adrenal cortex, but not in the mitochondria of other tissues. Cytochrome P-450, the liver microsomal drugmetabolizing enzyme is membranebound and fraction as multicomponent electron transport system for the metabolism of a variety of endogenous substrates(such as steroids, fatty acids, and bile acids) and exogenous substance (such as drugs, carcinogens, insecticides, and many other foreign compounds). The effect estrous cycle and preimplantation on the levels of hepatic microsomal cytochrome P-450 in mice were determined. In effect of estrous cycle on microsomal cytochrome P-450 level, the most increment was at estrus. The content of cytochrome P-450 was increased significantly at pregnant Day 3 of preimplantation.