1. Liver microsomal preparations from chick embryos (1 day before hatching) and from 1-7 day old chicks were assayed for oxidative drug-metabolizing activity with aminopyrine, aniline and naphthalene as substrates. 2. Activities for all three substrates were highest in preparations from 1 day-old chicks. These were more than twice as active as the 7 day-old preparations and about three times as active as those from the embryos. 3. The increase in drug-metabolizing activities in newly-hatched chicks was the same for either sex and persisted for 3 days before declining towards the 7 day-old levels. 4. The developmental time-course fo the liver microsomal drug-metabolizing activities was independent of any factor in the 105 000 g supernatant fractions and of such microsomal parameters as cytochrome b5 and cytochrome P-450 content, and NADPH-cytochrome c reductase activity, but was related to changes in NADPH-cytochrome P-450 reductase levels. 5. Treatment of 7 day-old chicks with exogenous inducers, 3-methylcholanthrene or phenobarbital sodium (100 mg/kg, intraperitoneally) brought about maximal stimulation of microsomal activity as 18-24 h. The time-course of this induction was reflected by changes in microsomal cytochrome P-450 content and NADPH-cytochrome P=450 reductase activities. 6. Some induction of liver microsomal drug metabolism in 7 day-old chicks could also be brought about by injecting certain lipid-soluble egg yolk extracts.