1. The response of the forearm resistance vessels to local intra-arterial infusion of verapamil and sodium nitroprusside has been assessed by a plethysmographic method in 23 men with normal arterial pressure and 35 men with primary hypertension.

2. In the 20 patients with hypertension for whom full dose—response curves were determined, the dilator response to verapamil was significantly greater than that in the normal controls, whereas the response to sodium nitroprusside was reduced.

3. Comparison of responses to the two dilators in individual subjects enabled the scattering effect of variables common to both drugs to be reduced. Analysis of the results in this way showed a highly significant difference between the hypertensive men as a whole and the normotensive controls (P < 0.00001); 24 out of the 35 hypertensive men showed responses that were outside the normal range.

4. The abnormal pattern of response observed in the hypertensive group cannot be accounted for by structural changes in the vessels and strongly suggests a functional abnormality of the vascular smooth muscle.

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