Chronic exposure of 3T3 mouse fibroblasts to insulin or to the glucocorticoid dexamethasone induces down-regulation and up-regulation, respectively, of cell-surface and total cellular insulin binding capacity. Both processes are reversed upon withdrawal of the inducer. Scatchard analysis of insulin binding for receptors in the down- and up-regulated states indicates that the changes in binding capacity result primarily from alterations in insulin receptor level. That these alterations in total receptor level are due to changes in cell-surface receptor level is indicated by the fact that the level of trypsin-insensitive, presumably intracellular, insulin binding sites does not change appreciably upon down- and up-regulation. The effects of insulin-induced down-regulation and dexamethasone-induced up-regulation on the rates of insulin receptor synthesis and decay were assessed by the heavy-isotope density-shift technique. Cells were shifted to medium containing heavy (2H, 13C, 15N) amino acids and, at various times after the shift, light and heavy receptors solubilized from total cellular membranes were resolved by isopycnic banding on density gradients and then quantitated. It was demonstrated that the insulin- and dexamethasone-induced alterations in insulin receptor level were due entirely to changes in the rate constant for receptor inactivation. The decrease in the first-order rate constant for receptor decay caused by dexamethasone is unexpected in view of the known action of steroid hormones in the induction of the synthesis of specific proteins.