A light microscopic study and cell counts of the inferior olivary nucleus (ION) were performed in the Purkinje cell degeneration mutant mouse (pcd/pcd). Six groups of animals aged from 24 to 210 days were studied and compared to age matched (+/+) C57Bl.6J mice. A cell deficit of about 30% was found in the youngest animals studied; it did not change significantly with age. The deficit affects the 4 subnuclei of the ION, but predominates clearly in the medial accessory olive. This cell loss is already established at the moment when the massive loss of Purkinje cells that has been reported in this mutant occurs. Thus it is not yet known if the ION represents a primary site of gene action or if the deficit is secondary to another cellular event, presumably the loss of Purkinje cells.