The effects of NGF, BDNF, NT-3, BDNF plus NT-3 and LIF on substance P (SP) mRNA levels were analysed in axotomized dorsal root ganglia (DRGs) in vivo by quantitative in situ hybridization. The growth factors were applied on to the transected sciatic nerve. SP mRNA levels were decreased significantly 3 days after axotomy. NGF (1 microgram) fully counteracted the down-regulation of SP mRNA. Neither BDNF (10 micrograms) nor NT-3 (10 micrograms) alone had any effect. However, co-administration of BDNF together with NT-3 (10 micrograms) distinctly reversed the decrease in SP mRNA. A similar effect was seen with a high (1.5 micrograms) but not a low dose of LIF (0.15 microgram). Our data suggest that the present method of delivering growth factors to the transected sciatic nerve is a valid way to study in vivo effects of growth factors on peptide expression in DRGs. Moreover, SP expression is regulated by several growth factors in vivo such as NGF, BDNF plus NT-3 as well as the neuroimmune factor LIF.