The functional role of brain insulin and insulinlike growth factor (IGF) receptors is being sought. Recently it has been found that these ligands are members of a newly identified family of neuritogenic polypeptides. We studied the relationship between 1251insulin and '251-IGF binding and their capacity to enhance neurite formation in cultured human neuroblastoma SH-SYSY cells. The binding of '251-insulin was temperature-dependent and heterogeneous. The Scatchard plot and dissociation rate were both consistent with the presence of two types of sites. There appeared to be about 900 high affinity sites per cell with a Kd of about 3 nM. This compared favorably with the half-rnaximal concentration of 4 nM for enhancement of neurite formation. The type I IGF sites were also present. Physiologic concentrations of insulin clearly enhanced neurite formation through the insulin sites, whereas physiologic concentrations of IGF-I and IGF-I1 enhanced through the IGF sites. Cross-occupancy of sites was observed at supraphysiologic concentrations, providing a reasonable explanation for the broad dose-response curves for these ligands. These results support the suggestion that one function of insulin and IGF receptors in neural tissues may be to modulate neurite formation.