The present study describes the development of an in situ hybridization histochemistry (ISHH) procedure which was employed to selectively monitor cellular distributions of the 2 major alternatively spliced beta- and gamma-species of mRNA encoding preprotachykinin (PPT) molecules found in rat CNS. For these purposes, 2 custom-designed oligodeoxynucleotide probes were synthesized corresponding to complementary sequences of beta- and gamma-PPT mRNAs. In particular, the gamma-selective probe was demonstrated to hybridize to the contiguous regions of RNA flanking the splice site formed by exclusion of exon 4. Initially, Northern blot analyses performed in conjunction with appropriate specificity controls demonstrated selective hybridization of the 32P-labeled beta- and gamma-selective probes to single bands of approximately 1.2-1.3 kilobases in size, consistent with previously established values for rat brain beta- and gamma-PPT mRNAs. In anatomical studies, results obtained from absorptions using competing nonradiolabeled oligonucleotides defined the specificity and selectivity of both probes for targeting their respective species of mRNA immobilized within sections of brain tissue. Extensive ISHH analyses using both beta- and gamma-selective probes demonstrated similar patterns of cellular labeling in all of the examined CNS areas. In addition, data obtained from analyses of adjacent thin sections of the dorsal root ganglia (DRG) indicated that beta- and gamma-PPT mRNAs were colocalized within individual DRG neurons, thereby suggesting generalized coexpression at the cellular level of both forms of mRNA. These data were complemented by semi-quantitative analyses which yielded cellular or intrinsic molar ratios of beta- to gamma-PPT mRNA of approximately 1:2-1:3, consistent with those values previously determined by nuclease protection analyses. In sum, a reasonable hypothesis evolving from the anatomical studies in combination with previous biochemical data supports the existence of a strong homeostatic mechanism involved in the maintenance of relatively constant intrinsic molar ratios of beta- to gamma-PPT mRNA by tachykinin-expressing neurons. The biological relevance of this putative fundamental relationship is discussed in the context of posttranslational processing of PPT molecules and of expression of mature tachykinins.