Thirty hypertensive outpatients of both sexes having diastolic blood pressure below 110 mmHg were included in a randomized trial. Their mean age was 50.4 +/- 8.6 years. After a two-week placebo period, the patients were treated with chlorthalidone in a single daily dose of 25 or 12.5 mg over the one-month period, and the next month the alternative dose (i.e. 12.5 or 25 mg) was administered. Changes in vascular reactivity were measured by occlusive plethysmography (mercury strain-gauge) at the end of the placebo period, and then post the first and second month of therapy. All the observed parameters, i.e. rest flow (RF), peak flow (PF), venous capacity (VC), and maximal venous outflow (MVO) increased with 25 mg of chlorthalidone, and similar findings were registered when a dose of 12.5 mg was given. There was a significant increase in venous capacity of both groups (from 2.6 to 3.2 and from 3.0 to 3.4 ml/100/min) and in peak flow of the group that was on 25 mg of chlorthalidone as the first dose (from 18.6 to 23.9 ml/100ml/min). Both doses had similar antihypertensive effect. It is concluded that antihypertensive effect of chlorthalidone is partly due to changes in vascular reactivity in the sense of vasodilation which is more prominent at the venous side of the blood flow, Low-dosed chlorthalidone is equipotent in antihypertensive efficiency, its side effects are rare, and the cost of therapy is by far the lowest.