To investigate a possible action of insulin on the rat kidney papilla, the binding of 125I-insulin to papilla microsomes was examined. This binding was specific to insulin in that it was displaced by increasing concentrations of unlabelled porcine insulin and to a lesser extent by porcine proinsulin and IGF-I, but not by IGF-II and bGH. Scatchard plot of the binding data was curvilinear consistent with either two classes of receptors with different affinities or a single class of receptors that showed negative cooperativity. A small fraction of 125I-insulin (maximum 2%) was degraded during incubation, but with a Km two order of magnitude higher than the constant of affinity for binding. Insulin stimulates the incorporation of phosphate to phosphatidylcholine in a dose-dependent manner, reaching a maximum with 10 nM insulin. This data showed both the presence of specific insulin receptors in the kidney papilla and an insulin action through the synthesis of phospholipids by insulin.