Low fibrinolytic activity, as measured by euglobulin (EFA), has been observed in obese subjects, and hypofibrinolysis may play a role in the pathogenesis of atherosclerosis and its complications. Blood fibrinolytic activity is regulated through a complex system of activators and inhibitors, especially plasminogen activator inhibitors (PA Inhibitors). In a group of 35 non-diabetic subjects with a wide range of body mass index (BMI), EFA was negatively correlated, and PA Inhibitor activity positively correlated, with BMI and plasma insulin levels. In a population of 49 non-diabetic obese women (differing from a control group of normal weight by lower EFA and higher level, of PA Inhibitor activity, plasma insulin and triglyceride), the PA Inhibitor activity was positively correlated with BMI, insulin and triglyceride. The increase in PA Inhibitor activity was associated with a high value of PA Inhibitor 1 antigen measured by an immuno-radiometric assay, indicating that the increased activity was due to a high level of circulating PA Inhibitor 1. Plasma insulin was lowered in obese non-diabetic subjects, without modification of the body weight, by a 24 hour fast or by treatment with Metformin. After 24 hours' fast, ten obese subjects had lower levels of insulin and PA Inhibitor activity and an increase in EFA. Treatment for 15 days by 1.75 g Metformin (or placebo), on a weight maintaining diet, induced, in the Metformin group, a decrease in plasma insulin, triglyceride and PA Inhibitor activity and an increase in EFA, while no change was observed in the placebo group.(ABSTRACT TRUNCATED AT 250 WORDS)