The cellular localization of substance P/neurokinin A-encoding preprotachykinin mRNAs in the rat enteric nervous system was studied by means of in situ hybridization histochemistry using 35S- or 3H-labeled single-stranded ribonucleic acid (RNA) probes which recognize all three preprotachykinin mRNA species, alpha, beta, and gamma. Substance P/neurokinin A-encoding mRNAs are expressed in neurons within the myenteric plexus of the esophagus and stomach, being more numerous in the latter, and in ganglion cells distributed to both the myenteric and submucosal plexuses of the intestine. Specificity of the hybridization was demonstrated by the lack of specific signal above background in sections incubated with a sense RNA probe or pretreated with ribonuclease A before hybridization. Ribonucleic acid blot hybridization analysis of RNA extracts from both the muscle layer-myenteric plexus and submucosal layer preparations of the duodenum demonstrated a single band of hybridization at 1.3 kb. Solution hybridization-nuclease protection assays showed multiple preprotachykinin-encoding transcripts in these RNA extracts, with an abundance level of gamma-mRNA greater than beta-mRNA much greater than alpha-mRNA, which is similar to that observed in the rat brain. Our results indicate that the preprotachykinin gene encoding the tachykinin peptides, substance P and neurokinin A, is transcribed in a population of enteric neurons that have a regional distribution comparable to the previously described tachykinin-like immunoreactive neurons, suggesting that specific mRNAs and the posttranslationally processed peptides are localized in the same structures.