The effect of acute and chronic colchicine treatment on glucose-induced immunoreactive insulin secretion and glucose tolerance was examined in fasting, unanesthetized, and unrestrained rats with indwelling jugular vein and aortic catheters. In the first study, intravenous glucose tolerance tests were performed by rapidly administering a glucose pulse (150 mg. intravenously) one hour after acute treatment with colchicine (0.5 mg. per kilogram of body weight, intravenously) or vehicle (control). Acute colchicine treatment caused significant suppression of glucose-induced insulin secretion and also markedly impaired glucose disappearance rates. In the second study, chronic colchicine treatment (in the lower dose of 0.2 mg. per kilogram of body weight, intraperitoneally, daily for 10 days) caused significant suppression of biphasic insulin secretion in response to a primed-constant glucose infusion (150 mg. of glucose pulse followed by 6 mg. per minute constant glucose infusion for 60 minutes) as compared with control rats (vehicle injection, intraperitoneally, daily for 10 days). Due to this marked decrease in biphasic insulin secretion, serum glucose concentrations were significantly higher between 10 and 45 minutes in the colchicine-treated rats than in the control rats. Therefore, acute and chronic colchicine treatment causes marked inhibition of glucose-induced insulin secretion and impairment of glucose tolerance in the intact rat. These observations suggest there is a need to evaluate carbohydrate metabolism in patients receiving colchicine treatment.