Bovine adrenodoxin mRNA is found to consist of several distinct mRNA species which can be divided into two sets. Each set utilizes at least three of four separate poly(A)+ addition sites providing an explanation of the three sizes of adrenodoxin mRNA (1.75, 1.4, and 0.95 kilobases) previously observed in bovine adrenocortical RNA by this laboratory (Okamura, T., John, M.E., Zuber, M.X., Simpson, E.R., and Waterman, M.R. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 5705-5709). The two sets are distinguished from one another by a unique 5' sequence leading to two different amino acid sequences (approximately 10% homology) for the precursor portion of this nuclear encoded, mitochondrial protein. A common mature adrenodoxin sequence is encoded by both sets of mRNA. One set of RNAs is 10-fold more abundant than the other, but the levels of both sets can be induced by treatment of primary bovine adrenocortical cell cultures with adrenocorticotropin. The biological significance of these two types of adrenodoxin precursor sequences remains obscure.