Recombinant plasmids specific for bovine adrenocortical cytochromes P-450sccand P-45011βhave been identified and characterized. Using these cDNA inserts as probes, it is found that tissue specificity of gene expression for these two proteins is as expected. Cytochrome P-450 mRNA is found in adrenocortical and corpus luteum RNA while cytochrome P-45011βmRNA is found only in adrenocortical RNA. Neither mRNA was detected in heart, liver or kidney RNA. Cytochrome P-450sccmRNA is found to be 1.9 kb in length while cytochrome P-45011βmRNA is found to be 4 kb in length. Treatment of bovine adrenocortical cells with ACTH or other modulators results in increased levels of cytochrome P-450sccand cytochrome P-45011βmRNAs. Cytochrome P-450sccis found to be encoded by a limited number or even a single gene.