To study the effect of hydroxychloroquine (HCQ) on glucose and insulin homeostasis, healthy rats were dosed with160 mg·kg-1·day-1 of HCQ orally, and streptozocin-induced diabetic rats received 80, 120, and 160 mg·kg-1·day-1 of HCQ, while controls received normal saline. Ten days after treatment with HCQ, healthy animals were challenged intravenously with insulin or glucose, while diabetic rats were given only an i.v. injection of insulin. In healthy rats, the areas within and under the glucose concentration - time curve following insulin and glucose challenge were estimated. In diabetic animals, the areas under the curve for both the percent change in serum glucose from baseline (AUG) and the percent change in serum insulin from baseline (AUI) were used as pharmacodynamic end points. In healthy rats, HCQ did not influence fasting serum glucose concentrations or glycemic profiles following i.v. administration of glucose or insulin. In diabetic rats, AUG and AUI were increased dependent on blood HCQ concentrations. The normal homeostatic mechanisms responsible for insulin-glucose regulation may compensate for possible HCQ-induced reduction of insulin metabolism in healthy rats. The HCQ dose- or concentration-effect relationships for glucose and insulin were linear over the range of HCQ concentrations tested. It is concluded that HCQ significantly elevated insulin blood concentration resulting in reduced glucose levels in a concentration-dependent fashion in diabetic rats. HCQ may have therapeutic potential in the treatment of type I and type II diabetes.Key words: hydroxychloroquine, enantiomers, insulin, glucose, diabetes, pharmacokinetics.