The nucleic acid and derived amino acid sequences of a chicken phenobarbital-inducible cytochrome P450 cDNA clone, pCHP7, are presented. The amino acid sequence shares 92% identity with that of a previously characterized chicken cytochrome P450 cDNA clone, pCHP3. The two clones, pCHP7 and pCHP3, represent two distinct mRNAs of 2.2 and 3.5 kb and appear to be transcribed from separate cytochrome P450 genes. Sequence comparisons with P450s from mammalian species indicate that the chicken P450s are most closely related to the IIC subfamily. According to the cytochrome P450 nomenclature (Nebert et al., 1989), the chicken P450 gene encoding the 3.5-kb mRNA is termed P450IIF1 and the P450 gene encoding the 2.2-kb mRNA is designated P450IIF2. When chick embryos were treated with 2-allyl-2-isopropylacetamide, the levels of the 3.5- and 2.2-kb P450 mRNAs in liver were elevated maximally by about 100-fold, while phenobarbital treatment resulted in a maximal increase of about 50-fold. These increases in mRNA levels were accompanied by a less than sixfold increase in the corresponding gene transcription rates. The results indicate that the increase in the amount of these two mRNAs is due to both transcriptional activation of the P450 genes and to a marked post-transcriptional mechanism. By contrast, the drug-induced hepatic mRNA levels for 5-aminolevulinate synthase, the rate-controlling enzyme of the heme biosynthetic pathway, could be accounted for predominantly by activation of gene transcription. A close correlation was observed between the time courses of hepatic mRNA accumulation for cytochrome P450 and 5-aminolevulilnate synthase following drug treatment of chick embryos. In adult hens it was demonstrated that a tissue-specific drug induction of cytochrome P450 mRNAs occurred with levels being substantially elevated in the liver, kidney, and small intestine. The mRNA for 5-aminolevulinate synthase was also drug-induced in the same tissue-specific fashion. The results are compatible with the gene for 5-aminolevulinate synthase being activated in response to an increased cytochrome P450 heme requirement.