The cerebellar cortex of mice aged 6, 15, 22, 25, 28 and 31 months was examined in parasagittal sections using quantitative histological techniques. The number Purkinje cells per mm declined from 13.0 +/- 0.2 at 6 months to 9.1 +/- 0.8 at 31 months. Granule cell density remained constant (2.63 x 10(6) per mm3) between 6 and 31 months of age. The granule cell to Purkinje cell ratio increased from 132.4 +/- 4.2 at 6 months of age to 184.2 +/- 6.9 at 31 months of age. The stellate and basket cell density declined from 83.8 +/- 8.1 per 10(6) mm at 6 months to 58.4 +/- 5.7 at 31 months. The stellate and basket cell to Purkinje cell ratio remained constant at 5.0 +/- 0.1 from 6 to 31 months of age whilst the granule cell to stellate and basket cell ratio increased from 25.5 +/- 0.6 at 6 months to 35.1 +/- 0.6 at 31 months of age. Since granule cell number does not increase in the adult the increase in the granule cell to Purkinje cell and granule cell to stellate and basket cell ratios must be due to loss of Purkinje cells and stellate and basket cells with increasing age. The correlation between Purkinje cell and stellate and basket cell loss is statistically significant (P less than 0.01).