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Insulin and insulinlike growth factors (IGF) are related polypeptides that have similar biological activities. Both factors produce metabolic effects as well as growth effects. Most cells have separate receptors for IGF and insulin. In the present study we have demonstrated specific IGF receptors in cultured smooth muscle cells of rat aorta. The properties and specificity of these receptors were compared with those of the insulin receptor in the same cell system. The specific binding of both 125I-IGF and 125I-insulin binding in the order of potency: insulin proinsulin IGF. The potency of IGF in displacing 125I-insulin was rapid and reversible. Maximal 125I-IGF binding occurred at 20 degrees C with a specific binding of 10%. At this temperature specific 125I-insulin binding was 1.3% and occurred in biphase. The pH optimum for 125I-IGF binding was between pH 7 and 8. Both receptors show a high degree of specificity. IGF, insulin and proinsulin competed for 125I-IGF binding in the order of potency: IGF proinsulin insulin. The potency of insulin in displacing 125I-IGF was about 2000 times lower than that of IGF itself. In addition, IGF, insulin and proinsulin competed for 125I-insulin was about 450 times lower than that of insulin. These results indicate two separate binding sites for insulin and IGF on arterial smooth muscle cells.