We examined insulin-like growth factor I (IGF-I)-dependent phosphorylation and protein tyrosine kinase (PTK) activity in cerebellar cortex of normal humans, patients with olivopontocerebellar atrophy (OPCA) ("C" kindred) and in lurcher mutant mouse, a suggested animal model for OPCA. PTK activity and IGF-I-dependent protein tyrosine phosphorylation was significantly reduced in cerebellar cortex of human OPCA patients as compared to the normal controls. Immunoblot analysis also demonstrated a decrease in cerebellar 80 kDa phosphotyrosine protein in these patients. By autoradiography, IGF-I receptors were localized in the molecular layer of 30-day-old control and lurcher mutant mice cerebella. However, the lurcher mutant mice showed a decrease in [125I]-IGF-I binding in the molecular layer as compared to the littermate controls. The IGF-I receptor autophosphorylation was also markedly reduced in 15-day- and 22-day-old lurcher cerebella. These results suggest that the process of cerebellar degeneration in human OPCA and lurcher mutant mouse may be associated with altered IGF-I receptor binding and protein tyrosine phosphorylation.