Calbindin D-28k and parvalbumin are neuronal calcium binding proteins of interest in relation to neurodegenerative diseases. Expression of calbindin and parvalbumin may be one of the determinants of selective vulnerability in these disorders. The distribution of these proteins was surveyed in the normal human motor system and in motor neuron disease (MND) using immunocytochemistry in formalin fixed post-mortem tissues. CNS tissues from 14 MND patients (mean age 61.2 years, mean post-mortem delay 24.6 h) and seven controls (mean age 62.6 years, mean post-mortem delay 25.3 h) were studied. Preliminary studies on the effects of fixation were performed. In normal cases upper and lower motor neurons showed absent expression of both proteins. Several neuronal groups characteristically spared in MND showed varying patterns of immunoreactivity: oculomotor neurons showed parvalbumin staining of the perikaryon; the thoracic preganglionic sympathetic neurons showed calbindin staining in perikarya. Onuf's nucleus showed calbindin staining in the neuropil only. In motor neuron disease a loss of ventral horn interneurons and calbindin immunoreactive processes was observed with no other disease related changes in the spinal cord, brain-stem, or motor cortex. These findings are consistent with the hypothesis that the distribution of these proteins is one determinant of selective vulnerability to the neurodegenerative processes in MND acting via disturbance of neuronal calcium homeostasis.