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Dogs with indwelling polyethylene arterial and venous catheters ran on a treadmill (slope 15 per cent, speed 100 m./min.). Diabetes was produced by alloxan or by a combination of alloxan and streptozotocin. Glucose turnover was measured according to the primed constant-rate infusion technics with 2-3H-glucose as tracer. In resting diabetic dogs plasma glucose varied between 200 and 650 mg./100 ml. There was a direct linear correlation between the hepatic glucose output (Ra) and the plasma glucose level. Exercise increased both Ra and the clearance rate (CR) of glucose; however, Ra could not match the rate of disappearance (=renal loss plus glucose uptake of the muscle), causing the plasma glucose to decline more rapidly than in the running control dogs. Two to three daysʹ treatment with methylprednisolone (MP, 3-3.2 mg./kg./day) caused a higher resting glucose level and a higher Ra. Exercise greatly increased the plasma glucose concentration, partly because MP enhanced the hepatic response but mainly because it essentially prevented the rise of CR. It is concluded that (a) in chemically induced diabetes, the variable glucose level is the result of a variable rate of hepatic glucose output; (b) the increase of Ra by MP treatment does not increase the plasma glucose in the normal dog but significantly aggravates the alloxan diabetes, (c) diabetes reduces the effect of exercise on the glucose uptake of the muscle, and this effect is potentiated by the inhibitory action of the glucocorticoid. This latter becomes unmasked only when the insulin secretion is impaired.