The activated form (p65) of nuclear factor kappa B (NF-kappa B) was found to be expressed in a sub-population (32%) of mixed diameter rat sensory neurons in L4 and L5 dorsal root ganglia, indicating that this transcription factor is involved in intracellular signalling in sensory neurons under physiological conditions. Four hours after crushing the sciatic nerve, ipsilateral p65 staining was abolished in a subgroup (60-70%) of these neurons. The contralateral side was unaffected by the injury and loss of NF-kappa B activity was not observed following sham surgery. Within 24 h of sciatic injury, ipsilateral p65 staining had recovered to control levels. We suggest that the decline in p65 after nerve injury was due to failure of retrograde axonal transport of trophic factor(s). Since neurotrophins and cytokines promote the survival of non-neuronal cells by activation of NF-kappa B, we believe that p65 may be critical for the resistance to apoptosis shown by adult sensory neurons.