We have investigated <em>17</em> alpha-hydroxylase and C<em>17</em>,20-lyase activities and the presence of cytochrome P450c<em>17</em> mRNA in the esophagus, stomach, duodenum, and colon of adult rats of both sexes. All tissues converted [4-14C]pregnenolone mainly to dehydroepiandrosterone (DHEA) through the 5-ene-3 beta-hydroxysteroid route as opposed to the 4-ene-3-<em>ketosteroid</em> pathway in a control testicular incubate. Synthesis of dehydroepiandrosterone was particularly high in the duodenum and was found to be lower in the stomach, colon and esophagus, in decreasing order. 20 alpha-Hydroxypregnenolone and progesterone were also formed primarily by the esophagus and colon, respectively. P450c<em>17</em> mRNA was demonstrated by ribonuclease protection assay in the stomach and duodenum, but not in esophagus and colon. However, a 335 bp-long cDNA fragment, whose sequence corresponded to that of rat P450c<em>17</em> cDNA, was amplified by reverse transcription (RT) and polymerase chain reaction (PCR) from the poly(A)+ RNAs of all four tissues. This result was further confirmed by Southern blotting using a 794-bp testicular probe. The complete sequence of P450c<em>17</em> cDNA in the stomach and duodenum was identical to that reported for rat testis P450c<em>17</em> cDNA. No amplification and no positive signal in Southern blotting were observed with the total RNAs from adult male adrenal and spleen, which were taken as negative controls since they had been previously found unable to form androgens from pregnenolone. Although the levels of transcription in gonads, duodenum and stomach were found to be equivalent, as indicated by the RNase protection assay and semiquantitative RT-PCR assay, P450c<em>17</em> enzyme activity was much higher in the testis, pointing at a possible dissimilarity in the respective rates of mRNA translation. Thus, P450c<em>17</em> is differentially expressed in the rat gastrointestinal tract, where it leads to the synthesis of the sex steroid precursor DHEA, especially in the duodenum and stomach.