In order to investigate a possible relationship between serotonergic and fibrinolytic systems, serotonin (5-hydroxytryptamine, 5-HT) metabolism was modified by the use of a monoamine oxidase inhibitor (tranylcypromine: 10 mg/kg body weight) and its effect on the fibrinolytic system was analyzed. Tranylcypromine caused a rise in whole blood 5-HT with a simultaneous fall in 5-hydroxyindoleacetic acid. Euglobulin clot lysis time was significantly prolonged 1 and 4 h after injection and tissue plasminogen activator (tPA) activity declined 4 h following administration with a simultaneous increase in its inhibitor (PAI) activity. Blood 5-HT was negatively related to tPA (r = -0.48; p < 0.05) and positively to PAI activities (r = 0.43; p < 0.05) in tranylcypromino-treated rats, whereas there were no such correlations in solvent-treated rats.