By means of stereological techniques such as the optical disector, the total number of cells in the spinal cords of five albino mice were estimated. The mean number of cells in the gray and the white matter was 9.0 and 4.1 million, respectively. Of the cells in the gray matter, 4.0 million were classified as neurons, 2.8 million as glial and 1.7 million as endothelial cells whereas 0.4 million remained unclassified. Of the neurons in the gray matter, 1.2 million were located in the cervical region, 1.4 million in the thoracic region, 0.9 million in the lumbar region and 0.4 million in the sacro-coccygeal region. The gray matter in the spinal cord of the mouse contained relatively more neurons and less glial cells than the gray matter in the rat spinal cord. The fraction of cells in the cervical, thoracic, lumbar and sacro-coccygeal part of the cord were, however, quite similar in the two species. In both species the number of neurons in the ventral horns made up about 8-9% of the total neuron population in the gray matter of the cord.