Systemically-administered inorganic mercury localizes to motor neurons, but it is not known if mercury injures these neurons. We therefore looked for signs of damage to the motor and sensory neurons of mice that had been exposed to inorganic mercury. Young adult mice were injected intraperitoneally with either 1 or 2 microg/g of mercuric chloride and perfused 1 or 30 weeks later. The cellular distribution of mercury in the spinal cord was examined with silver nitrate autometallography. The numbers and sizes of myelinated axons in the L5 anterior and posterior roots were quantitated using an image analysis program. Mercury was found throughout the cytoplasm of motor neuron cell bodies after 1 week and in paranuclear aggregations after 30 weeks. Thirty weeks after exposure to either 1 or 2 microg/g of mercury, fewer large myelinated axons were seen in mercury-injected groups than in controls, though total numbers of myelinated axons did not differ between groups. A slight increase in numbers of small axons was seen in the posterior roots of mice exposed to 1 microg/g of mercury. In conclusion, inorganic mercury remains within mouse neurons for prolonged periods and causes a reduction in the size of myelinated axons in the anterior root and to a lesser extent the posterior spinal root. Inorganic mercury within motor neurons therefore appears to behave as a slowly-acting neurotoxin that shrinks motor axons.