A B S T R A C T We have tested the idea that the circulating plasma insulin level plays an important role in the long-term regulation, or maintenance, of the cellular glucose transport system, distinct from insulin's ability to acutely accelerate glucose transport. To study this hypothesis, groups of rats were made either hyperinsulinemic or hypoinsulinemic by daily insulin injections, or streptozotocin treatment, respectively. Different levels of hypoinsulinemia were produced by using different doses of streptozotocin (40 and 55 mg/kg). The mean (+SE) 9 a.m. plasma insulin level for each experimental group was: hyperinsulinemic animals, 65+5 ,uU/ml; controls, 32+3 AU/ml; low dose streptozotocin group, 18+3 ,uU/ml; and high dose streptozotocin group 5+2 ,uU/ml. Isolated adipocytes were prepared from each animal and glucose transport was assessed by measuring the initial rates of uptake of the nonmetabolyzable hexose 2-deoxy glucose. The Vmax and Km values for adipocyte glucose transport were calculated from the 2-deoxy glucose uptake data. The hypoinsulinemic rats. Furthermore, when the individual data were analyzed, highly significant correlation coefficients were found between the height ofthe plasma insulin level and both the basal (r = 0.82, P < 0.001) and insulin-stimulated (r = 0.93, P < 0.001) Vmax values. The apparent Km for 2-deoxy glucose uptake was the same under all conditions.In conclusion, assuming that the Vmax of transport is some function of the number of glucose transport carriers per cell, then these results support the hypothesis that in addition to acute acceleration of glucose transport, insulin is also an important long-term regulator of the number of available adipocyte glucose transport carriers.