In this study we examined whether the reduced fibrinolysis and increased platelet activity that are known to occur in hypertension are already present in borderline hypertension. Twelve patients with 'borderline' hypertension (diastolic blood pressure 90-95 mmHg) were found to have substantially reduced fibrinolytic activity, both at rest and during exercise, compared with 12 normotensive controls. Euglobulin clot lysis time (ECLT) was significantly higher in hypertensive subjects (218 min vs. 145 min; P < 0.05), and this difference persisted during exercise. Resting tissue plasminogen activator activity (t-PA) did not differ in the two groups, but the brisk increase in t-PA in controls during exercise (0.64 rising to 1.44 IU mL-1; P < 0.01) did not occur to the same extent in the borderline hypertensive subjects. Levels of the fast-acting t-PA inhibitor, normally referred to as PAI-1, were considerably higher in hypertensives (9.22 vs. 4.41 IU mL-1; P < 0.02), and this difference persisted in the upright posture, indicating a decrease in fibrinolytic activity. Platelet aggregability induced by ADP in vitro was not significantly higher in the hypertensive subjects, but indices of platelet activity in vivo (B-TG and PF-4 levels) revealed enhanced platelet function in the hypertensives. These results indicate that the indicators of altered haemostatic function known to occur in hypertension, namely diminished fibrinolytic activity and increased platelet function, are already detectable during the very earliest stage of the disease.