Using in situ hybridization histochemistry, we examined the biosynthesis of the preprotachykinin-A (PPTA) mRNAs, which code for substance P (SP) and also a related peptide, substance K (SK), in the rat dorsal root ganglion (DRG). The animals were injected with a small amount of formalin in the right hindpaw for nociceptive chemical stimulation. Analysis of hybridization signals in the emulsion autoradiography showed that the lumbar-5 DRG neurons expressing PPTA mRNAs were significantly increased in number and signal intensity on the formalin-injected side. Such elevation of the PPTA mRNA expression in the DRG was observed as early as 3 h after formalin injection. These results show that the expression of the PPTA gene is activated by chemogenic noxious stimuli.