Corticosterone is the major circulating glucocorticoid in adult mice and rats, and this is explained, in part, by the absence of 17 alpha-hydroxylase cytochrome P450 (P450c17) in adrenal glands of these rodents. During embryonic development, however, we discovered transient expression of P450c17 in a subset of adrenocortical cells in the fetal mouse adrenal. This differs from cholesterol side-chain cleavage cytochrome P450 and adrenodoxin, which are expressed continuously in most fetal adrenocortical cells from onset of expression at embryonic days 11-12 (E11-12) until term. Adrenal P450c17 transcripts are detectable in situ at E12.5 and increase in abundance from E12.5 to E14.5. Transcripts are then lost between E16.5 and term (E18.5) and are undetectable in situ in adrenal glands of adult mice. These results are consistent with the presence of pregnenolone 17 alpha-hydroxylase activity in adrenal homogenates of fetal but not adult mice. By using polymerase chain reaction, we determined that murine fetal (E14.5-15.5) adrenal glands contain messenger RNAs (mRNAs) encoding all of the steroid hydroxylases required to produce cortisol and corticosterone but little aldosterone synthase mRNA. Adrenal glands from adult mice contain mRNAs encoding steroid hydroxylases required to produce corticosterone and aldosterone but not cortisol (little P450c17 mRNA). The spatial and temporal expression patterns of P450c17 and aldosterone synthase mRNA, which differ from those of cholesterol side-chain cleavage cytochrome P450 and adrenodoxin, suggest that multiple factors must be required to program cell type- and species-specific expression of these steroid hydroxylases during embryonic development.