The binding and biologic potencies of human biosynthetic proinsulin (HPro) were determined in isolated rat adipocytes. At both 16°C and 37°C, proinsulin was found to have 3% (on a molar basis) the potency of porcine insulin for displacing bound [(125I)TyrA14]-insulin from insulin receptors. Human biosynthetic proinsulin also had 3% of the molar potency of insulin for stimulation of deoxyglucose transport (EC50 = 8.8 ± 0.05 × 10−11 M for insulin and 2.9 ± 0.55 nM for HPro). However, both hormones produced the same maximal effect on glucose transport. In order to determine if the delay in onset and persistence of proinsulin action seen in vivo was due to any differences at the cellular level, the time course of HPro action on glucose transport was determined. Biologically equivalent submaximal concentrations of insulin (0.166 nM) and HPro (4.44 nM) gave identical time courses for stimulation of deoxyglucose transport at 37°C with half-maximal effects at 4 min and full effects by 30 min. Maximally stimulating concentrations of insulin (1.66 nM) and HPro (22.2 nM) also had superimposable time courses. Deactivation of stimulated glucose transport was determined by incubating equivalent concentrations of insulin (0.166 and 1.66 nM) and HPro (4.44 and 22.2 nM) until full stimulation was achieved, washing cells free of unbound hormone, and initiating dissociation and deactivation by resuspension in hormone-free buffer. Both the absolute activities of transport and rates of deactivation were the same for insulin and HPro. At the submaximal concentrations, 50% of the hormones' effects were lost by 20 min, while 50 min was required after maximal stimulation for 50% deactivation. Therefore, the activation and deactivation kinetics of HPro were no different at the cellular level than for insulin;the delayed onset and prolonged persistence of action in vivo may be due to differences in in vivo pharmacokinetic and/or metabolism.