The vestibular system helps the body to maintain equilibrium. There are four vestibular nuclei on the right and left sides, the medial vestibular nucleus being the largest. The volumes and total numbers of neurons in the left and right medial vestibular nuclei of adult male and female rats were estimated using stereological techniques. The volumes of the left and right medial vestibular nucleus were 0.67 +/- 0.03 mm(3) and 0.71 +/- 0.02 mm(3) in the female, and 0.55 +/- 0.02 mm(3) and 0.61 +/- 0.03 mm(3) in the male rats, respectively, Total neuron numbers in the left and right medial vestibular nuclei were 19.364 +/- 791 and 20.978 +/- 784 in the female, and 16.905 +/- 229 and 15.547 +/- 439 in the male rats, respectively, No asymmetry in volume was found between the left and right sides in either sex; but a significant difference in volume was observed for the right medial vestibular nucleus between male and female rats. A significant difference in total neuron number between the left and right medial vestibular nuclei was observed in female and male rats: in male rats, left > right; in female rats, right > left. There was also a significant difference between male and female rats with regard to total number of neurons in the medial vestibular nuclei, the female having more neurons than the male on both sides, that is, female > male. These results indicate that neuron number in the medial vestibular nucleus shows laterality in the same sex, and a female-based sexual dimorphism.