The capillary filtration coefficient (CFC) in the forearm was followed in patients with essential hypertension and compared with controls, using the method of venous occlusion plethysmography and simultaneous recording of intravenous pressure in the measured segment of the extremity. Arterial blood-pressure was obtained in the contralateral arm by auscultation. The CFC averaged 0.0039 +/- 0.009 ml/100 ml/min/mmHg and 0.0095 +/- 0.0018 ml/100 ml/min/mmHg in hypertensives and controls respectively. The relation between CFC and arterial blood-pressure values was inversely non-linear. The results suggest reduction of the capillary area due to hypertension and possibly functional or structural changes in the microcirculatory bed accompanying systemic blood-pressure increase.