Gender differences have been noted in the tissue distribution of mercury. We sought to determine if the uptake of low-dose inorganic mercury into motor neurons differs between male and female mice. Four male and four female mice were injected i.p. with 0.5 mg HgCl2/kg. In 50-microns sections of lumbar spinal cord stained with autometallography, six motor neuron cell bodies were selected for study. The volume percentage of mercury granules in the cell bodies was estimated using a confocal microscope. Mercury granules occupied more perikaryal volume in motor neurons from female mice (mean 3.7%) than from male mice (mean 2.2%) (p < or = 0.05). After the same dose, the amount of renal mercury measured by mass spectrometry was significantly less in six female than five male mice. In conclusion, female mice take up more inorganic mercury into their motor neurons than do male mice. This may be related to a smaller deposition of mercury in the female kidney, leaving more circulating mercury available to be taken up by motor axons.