50g oral glucose tolerance test (50g OGTT) was performed on 61 hospitalized patients suffered from ischemic or hypertensive heart disease (without acute stage of myocardial infarction, congestive heart failure, renal or hepatic disorder, secondary hyperlipidemia, endocrine disorder, etc.) to elucidate the significance of plasma free fatty acids (FFA) in men fed 1, 000 calory formula diets. The subjects were divided into four groups according to basal FFA level (maximum decreased level of FFA during 50g OGTT) with or without glucose intolerance.Results: In the groups with normal glucose tolerance the different level of basal FFA caused the characteristics as follow. Relatively high basal FFA (over 391μEq/l) accompanied with significantly high response of insulin to 50g oral glucose (123±57μU/ml to 37±21μU/ml for low basal FFA) in spite of low frequency of delayed maximum insulin and low insulinogenic index. This subgroup also had high relative body weight before the calory-restriction though there was not seen in different response of body weight to calory-restriction.These results, especially the increase of basal FFA after calory-restriction seen in the subjects with high response of insulin, might be estimated the different action of insulin between glucose utilization and the inhibition of FFA release in obesity.In the groups with abnormal glucose tolerance high basal FFA was not shown only hyperresponse of insulin to 50g oral glucose but also obesity.These results are discussed on the content of the possible role of plasma FFA in obesity and glucose intolerance.