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Using the reverse transcription polymerase chain reaction, mRNAs encoding steroidogenic P450s as well as NADPH-cytochrome P450 reductase (P450 reductase), adrenodoxin and the transcription factor steroidogenic factor 1 (SF-1) were all detected in rodent brain, but their distribution between brain regions varied. Adrenodoxin and P450 reductase were detected in all regions, suggesting the presence of both mitochondrial and microsomal P450s throughout the brain. Messenger RNAs encoding P450scc (CYP11A1) and P45017 alpha (CYP17) were also detected in all brain regions, this being the first report of CYP17 in the brain. P450c21 (CYP21) was detected only in the brain stem. P45011 beta (CYP11B1) and P450aldo (CYP11B2) are expressed in rat brain, but not in mouse brain; CYP11B1 primarily in the cerebrum, whereas CYP11B2 was detected in all brain regions. In both species, highest levels of aromatase P450 (CYP19) mRNA were detected in the cerebrum. SF-1 expression was restricted to the cerebrum minus cortex. Thus, although SF-1 is required for high level expression of the steroidogenic enzymes in adrenals and gonads, other factors may influence the expression of these genes in the brain. If the mRNAs detected by RT-PCR are indeed translated into functional enzymes, these studies suggest that different brain regions have different capacities for local steroid hormone production and metabolism. This raises the technical challenge of locating the specific sites of synthesis as well as the function of such locally produced ligands.