General concepts of hepatic drug-metabolizing enzymes in liver microsomes and the characteristics of induction of the enzyme are well known in adult rats. However there are few detailed reports on the properties of the enzymes and characteristics of the induction in the rat fetus. The present investigation examined the effect of maternal consumption of alcohol during pregnancy and lactation on the activities of mixed function oxygenase of liver microsomes of the neonatal rats. The levels of the electron transfer system, such as cytochrome P-450, NADPH cytochrome C reductase were increased from 2nd week to 3rd week of postnatal age and then decrease on the alcohol treated group. The activities of 7-ethoxycoumarin-o-deethylase were increased until 3rd week of postnatal age and then decreased on the alcohol treated group compared to control group. These results suggested that induction of MFO were stimulated by alcohol were disturbed the liver microsomal drug metabolism.