The experiments were undertaken to determine if an antithyroid agent (propylthiouracil, PTU) and/or ascorbic acid (AA) affect mixed-function oxidase (MFO) in hepatic microsomes of male broiler chicks. 2. Feeding PTU increased the MFO activity in a dose-related manner. Addition of AA to the PTU-containing diet further increased the content of cytochromes P-450 and b5, but not the activities of NADPH-cytochrome c and NADH-cytochrome b5 reductase, and the drug-metabolizing enzymes. 3. Supplemental AA induced cytochrome b5 rather than cytochrome P-450.