The neuronal apoptosis inhibitory protein (NAIP) is known to have anti-apoptotic functions, and its gene is often mutated in severe cases of spinal muscular atrophy (SMA), a disease characterized by motor neuron degeneration. In this study, we examined the distribution of the endogenous NAIP protein in normal human spinal cord and brain tissue by using a polyclonal antibody against NAIP. Immunohistochemical staining demonstrated that NAIP is strongly expressed in anterior horn and motor cortex neurons of normal brains, and it is not altered in the remaining motor neurons of patients with amyotrophic lateral sclerosis (ALS). NAIP is also located in human fetal neurons and in adult choroid plexus cells. These results suggest that the antiapoptotic molecule NAIP may be important in motor neurons, but it specifically does not appear to be altered in ALS.