The regulatory effects of nerve growth factor (NGF) on tachykinin biosynthesis in rat primary sensory neurons during the period of postnatal development were examined under in vivo and in vitro conditions. Administration of NGF to neonatal rats led to a significant increase in protein levels of substance P (SP) and neurokinin A (NKA) in trigeminal and dorsal root ganglia (DRG). In addition, Northern blot analysis revealed that preprotachykinin mRNA was upregulated in sensory ganglia of neonatal animals after treatment with NGF. Using a well-defined in vitro system for neonatal rat DRG and trigeminal ganglia neurons, we found that addition of NGF induced SP and NKA protein levels in a dose-dependent manner. Furthermore, preprotachykinin mRNA was markedly increased in cultured DRG and trigeminal ganglia neurons in the presence of NGF. Thus, our results clearly demonstrate that NGF regulates tachykinin gene expression and biosynthesis both in vivo and in vitro during the developmental period of rat sensory neurons.