Forearm blood flow response to posture change in the very old: non-invasive measurement by venous occlusion plethysmography. Little is known about the peripheral vascular response to posture change in very elderly people who are vulnerable to the development of orthostatic hypotension. This is due, in part, to the risks of currently utilized invasive vascular monitoring techniques in the elderly population. We studied the forearm vascular response to active standing in 18 healthy young, 10 healthy old, and 19 impaired elderly subjects, using the non-invasive technique of venous occlusion plethysmography. In six subjects this technique was compared to duplex doppler ultrasonography for the measurement of postural changes in forearm blood flow. Forearm blood flow changes determined by venous occlusion plethysmography were 11% larger than doppler measurements, but the two methods strongly correlated (r = 0.90, P less than .001). Mean forearm vascular resistance increased to a significantly greater extent at 1 minute of standing in young subjects than in both groups of old, although the response was quite variable in all groups. Two healthy elderly (20%) and eight impaired elderly (40%) subjects had unexpected forearm vasodilatation at 1 minute of standing. By 3 minutes, forearm vascular resistance had increased by similar amounts in all three groups of subjects. Five impaired elderly and no healthy young or healthy old subjects had orthostatic hypotension, defined as greater than or equal to 10 mm Hg decline in mean arterial blood pressure at 1 or 3 minutes of standing. Forearm vascular resistance changes did not correlate with blood pressure response to standing. Thus, forearm vascular response to 1 minute of active standing is attenuated in many elderly subjects. This abnormality may impair adaptation to orthostatic stress in advanced age. 