Two overlapping cDNA clones (pBSCC-1 and pBSCC-2) bearing inserts approximately equal to 425 and approximately equal to 950 base pairs long, respectively, which are specific for bovine cholesterol side-chain cleavage cytochrome P-450 (P-450scc), have been identified by using two differential hybridization screening procedures followed by hybrid-selected RNA translation. By using these cloned cDNAs as hybridization probes, an RNA species was identified that had the properties expected of mRNA specific for P-450scc with respect to tissue specificity, corticotropin (ACTH)-mediated regulation of synthesis, and size of the protein product synthesized in vitro. In RNA samples obtained from bovine adrenal cortex, from bovine corpus luteum, and from cultured bovine adrenocortical cells, it was found that P-450scc is encoded by mRNA species approximately equal to 2000 bases long, a majority of which are polyadenylylated. P-450scc mRNA was not detected in RNA samples prepared from bovine heart, liver, and kidney. Treatment of cultured bovine adrenocortical cells with ACTH resulted in the appearance of elevated levels of P-450scc mRNA within 8 hr. Thus, ACTH promotes the enhancement of P-450scc gene transcription or acts to stabilize the transcripts. When pBSCC-2 cDNA was used to probe high molecular weight bovine DNA following treatment with restriction endonucleases, a simple pattern of hybridization was observed indicating that P-450scc may be encoded by a single gene.