Previous work indicates that a serum form of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/Man-6-P) receptor that circulates in mammals is a truncated form of the cellular receptor which lacks its cytoplasmic domain. In this study the effects of insulin administration on the levels of the serum forms of IGF-II/Man-6-P and transferrin receptors were measured. After sc injection of insulin into rats fasted overnight, the amount of IGF-II/Man-6-P receptor in serum increases to approximately twice that observed in untreated animals within 30 min, then decreases to a level lower than the initial level by 60 min before returning to control values by 90 min. The time course of the initial increase in serum receptor levels in response to insulin is similar to that observed for the decrease in blood glucose concentrations. Measurements of serum enzyme activities (creatine kinase and lactate dehydrogenase) during this time show no significant increase in response to insulin treatment. Furthermore, the increase in serum IGF-II/Man-6-P receptors is proportional to the amount of insulin injected over the range tested. In diabetic rats the serum IGF-II/Man-6-P receptor concentration is decreased to approximately 80% as much serum receptor as that in normal age-matched rats. The acute response of serum IGF-II/Man-6-P receptor levels to insulin administration is similar in both time course and extent of change to the increase in isolated fat cells of cell surface receptor levels due to insulin action. These results suggest the hypothesis that insulin stimulates the movement of cellular IGF-II/Man-6-P receptors to the cell surface, where they are proteolytically cleaved and released into serum.