This study was undertaken to obtain further information on the mechanism by which blood fibrinolytic activity, a balance between plasminogen activators and inhibitors, is lowered in obese subjects. Fasting blood samples were collected from 35 subjects, aged 15 to 45 years, with normal glucose tolerance and a Body Mass Index (BMI) varying widely between 16 and 45 (normal, 19 to 25). Euglobulin Fibrinolytic Activity (EFA) did not correlate with the level of tissue type plasminogen activator (t-PA) related antigen but exhibited a negative correlation with the level of PA inhibitor (r = -.609, P less than 0.01). EFA was negatively and PA inhibitor positively correlated with both BMI (r = -.381, P less than 0.02 and .664, P less than 0.01, respectively) and plasma insulin level (r = .410, P less than 0.02 and .521, P less than 0.01, respectively). Stepwise analysis showed that these correlations were independent. As expected, plasma insulin was correlated with BMI (r = .512, P less than 0.01) and triglyceride level (r = .38, P less than 0.02), total cholesterol with age (r = .379, P less than 0.02). Ten obese subjects were submitted to a 24-hour fast. While body weight did not change appreciably, plasma insulin decreased from 22.3 +/- 2.2 to 16.3 +/- 1.1 microU/ml, EFA increased from 3.6 +/- .8 to 4.9 +/- .67 mm, and PA inhibitor decreased from 4.52 +/- .76 to 3.44 +/- .63 IU/mL. All these differences were significant. T-PA-related antigen did not change.(ABSTRACT TRUNCATED AT 250 WORDS)