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To investigate the molecular basis for the pattern of ovarian steroid production during the bovine estrous cycle, the relative levels of mRNA specific for cholesterol side-chain cleavage cytochrome P-450, 17 alpha-hydroxylase cytochrome P-450, adrenodoxin, and low density lipoprotein receptor were determined in ovarian antral follicles of differing size (less than 3-18 mm) and corpora lutea from the early, early-mid, late-mid, and regressionary stages. Total and poly(A)+ RNA was size-fractionated on agarose-formaldehyde gels, transferred to nylon filters and hybridized to specific 32P-labeled probes. The levels of mRNAs for the rate-limiting enzymes in the conversion of cholesterol into progesterone, namely cholesterol side-chain cleavage cytochrome P-450 and its electron donor, adrenodoxin, were higher in corpora lutea than in follicles. Conversely the levels of mRNA specific for the key regulatory enzyme in the conversion of pregnenolone or progesterone to androgen, namely 17 alpha-hydroxylase cytochrome P-450, were high in all antral follicles examined but were low in young corpora lutea and undetectable in more mature corpora lutea. Low density lipoprotein receptor mRNA was detectable in antral follicles and corpora lutea but the levels were greater in corpora lutea. These results suggest that the pattern of changes in steroid hormone biosynthesis during the bovine estrous cycle and in the ovarian content of steroidogenic enzymes is related to and probably dependent upon the pattern of change in levels of mRNAs for steroidogenic enzymes and related proteins.