The numerical densities of granule and Purkinje cells in the cerebellar cortex of the rat were determined by stereological methods. The density of Purkinje cells in our fixed material was 1,018 +/- 39 per mm2 (mean +/- s.e.m.) of Purkinje cell layer and that of granule cells 1.92 +/- 0.03 x 10(6) per microliter of granular layer. The total area of Purkinje cell layer was 332 mm2 and the volume of granular layer was 48 microliters. The rat cerebellum therefore contains 3.38 x 10(5) Purkinje cells and 9.2 x 10(7) granule cells, from which there are 274 granule cells for each Purkinje cell. The density of granule cells and the density of parallel fibers in the molecular layer observed in a companion study indicate that the average length of a parallel fiber is around 5 mm.