Amyotrophic lateral sclerosis is a lethal neurodegenerative disorder involving motoneuron loss in the cortex, brainstem and spinal cord, resulting in progressive paralysis. Aberrant neurotrophin signalling via the low affinity neurotrophin receptor p75 has been suggested to be involved in the motoneuron death by the activation of apoptotic pathways. In order to investigate the involvement of neurotrophin receptor p75 in the amyotrophic lateral sclerosis related motoneuron degeneration process, we have studied the expression of this receptor in the spinal cord of transgenic mice carrying a mutated human Cu, Zn superoxide dismutase gene. Mutations in the superoxide dismutase gene are one of the genetic causes for familiar amyotrophic lateras sclerosis and human superoxide dismutase-1 transgenic mice develop symptoms and pathology similar to those in human amyotrophic lateras sclerosis. Our study shows that in these mice, spinal motoneurons, which normally do not contain the neurotrophin receptor p75 receptor, express this receptor during the progress of the disease. Expression of the neurotrophin receptor p75 receptor coincides with the expression of activating transcription factor 3, a member of the activating transcription factor/cyclic AMP family of stress transcription factors. Only a minority of these spinal motoneurons actually showed co-expression of neurotrophin receptor p75 with caspase-3 activity, suggesting that expression of the neurotrophin receptor p75 receptor is not directly related to the execution phase of the apoptosis process.