1. The dilator response to local infusion of K+ has been assessed in the forearm resistance vessels of 17 men with primary hypertension and 11 controls, by using a standard plethysmographic method.
2. The response to infusion of K+ at 0.1 mmol/min was smaller in the patients with hypertension than in the normal controls (P<0.03).
3. The results are consistent with the view that the activity of the sodium pump is depressed in the resistance vessels of patients with hypertension, but they yield no evidence as to whether or not this abnormality contributes to the elevation of the peripheral resistance.