To observe tbe effeet of moderate exercise on blood glucoee, free fatty acid, insulin and counter-regulatory hormones in diabetic patients, 4 diabetic men and 4 healthy men, underwent physical exercise on bicycle ergometer at VO,max 50% for 40 minutes. This results are as follows; 1) In recent onset diabetic patieots wlth mild fasting hyperglgcemia, the levels of free fatty acid and counter- regulatory hormones increa signifieaytly both during and after meiderate exercise. In recent onset diabetic patient with severe fasting hyperglpcemia, free fatty acid was elevated and the magnitude of increment of counter-regulatory hormones was Iess pronounced than in the patient with mild fasting hyyerglycemia. In diabetic patient of long duration with severe fasting hyperglyceinia, the rise of free fatty acid and counter- regutatory hormones were not pronounced. The rise of free fatty acid was pronounced after exercise in the patient who exhibited low blood glucose level during exercise. 2) The increase in free fatty acid concentration was correlated with the increase in catecholamine concentratioin during exercise, and accompanies with the increase in cortisol conceaitrstion after exercise. Glueagon increased slplificiantIy where free fatty acid decreased after exercise. 3) The level of insulin changed in accordance with that of free fatty acid both during and after exercise. This study suggests that the role of catecholamine is the most important for the mobilization of free fatty acid during exereise in type ll diabetic patients. And it is suggested that catcholamine response to exercise may vary considerbly respective of the degree of glucose control and the duration of illness, and cortisol and glucagon may play a role ia the mobilisation of free fatty acid and hepatic glueese production.