A density-shift method is described for analyzing insulin receptor synthesis and turnover in cultured cells labeled with "heavy" amino acids (2H, 13C, and 15N). Solubilized newly synthesized heavy and old "light" receptors are separated by isopycnic banding on CsCl gradients and then quantitated. Insulin receptor synthesis and turnover were studied by this technique in 3T3-L1 preadipocytes which undergo an increase in insulin binding capacity during differentiation. The results indicate that the increase in insulin binding capacity is a consequence of new receptor synthesis, that the insulin receptor has a relatively short half-life (6.7 hr), and that an increased rate of receptor synthesis contributes to the increase of insulin receptor level during differentiation.