Recent clinical studies have established that bone density is related to both fat mass and circulating insulin levels. A direct action of insulin on the osteoblast may contribute to these relationships. Osteoblast-like cells have insulin receptors, and insulin has been shown to stimulate proliferation of these cells in vitro. However, it has not been possible to study the effects of insulin administration on bone in vivo because of the metabolic effects of insulin, particularly hypoglycemia. A model involving the local injection of insulin over one hemicalvaria of an adult mouse overcomes these difficulties and permits the histomorphometric study of insulin's action on bone. Insulin or vehicle was injected daily for 5 days over the right hemicalvariae of adult mice, and the animals were sacrificed 1 week later. All indices of bone formation were significantly increased in insulin-treated hemicalvariae compared with the noninjected hemicalvariae. There was a 2.73 +/- 0. 50-fold increase in osteoid area (P = 0.005), a 2.20 +/- 0.37-fold increase in osteoblast surface (P = 0.021) and a 2.04 +/- 0.29-fold increase in osteoblast number (P = 0.012). Indices of bone resorption tended to decline and mineralized bone area tended to increase in insulin-treated animals. The direct action of insulin on bone may contribute to the increased bone density seen in obesity and to the osteopenia of type I diabetes, conditions associated with insulin excess and deficiency, respectively.