Insulin and insulin-like growth factor-I (IGF-I) are known to affect cardiovascular disease. We investigated ligand binding and the dose-response relationship for insulin and IGF-I on vascular smooth muscle cells (VSMCs) at the receptor level. VSMCs from rat thoracic aorta were serum starved, stimulated with IGF-I or insulin, lysed, immunoprecipitated (IP) and analyzed by Western blot. D-[U-¹⁴C]-glucose accumulation and [6-³H] thymidine incorporation into DNA were also measured. Specific binding of both insulin and IGF-I was demonstrated, being higher for IGF-I. Both IGF-I receptor (IGF-IR) and insulin receptor (IR) -subunits were detected and co-precipitated after IP against either of the two. No co-precipitation was found after reduction of disulphide bonds with dithiotreitol before IP. After stimulation with 10^-10-10^-9M IGF-I, IP of the IGF-IR or IR -subunit and immunoblot with anti-phosphotyrosine antibody (PY20), we found two distinct bands indicating phosphorylation of both the IGF-IR and the IR -subunit. Stimulation with 10^-10-10^-9M insulin and IP against the IGF-IR did not show phosphorylation of either -subunit, while after IP of the IR we found phosphorylation of the IR -subunit. ¹⁴C-glucose accumulation and ³H-thymidine incorporation was elevated in cells stimulated with IGF-I at 10^-10-10^-7M, reaching maximum by 10^-9M. Insulin stimulation showed measurable effects only at supraphysiological concentrations, 10^-8-10^-7M. In conclusion, co-precipitation of both the IGF-IR and the IR -subunit indicates the presence of hybrid insulin/IGF-I receptors in VSMC. At a physiological concentration insulin activates the insulin receptor, but does not affect either glucose metabolism or DNA-synthesis, whereas IGF-I both activates the receptor and elicits biological effect.