1. There is a biphasic flow response measured plethysmographically after release of prolonged venous occlusion of the forearm.
2. The response consists of an early, vasodilatory, increase in flow and is followed by a decrease in flow relative to control, thought to be mediated by myogenic contraction of resistance vessels.
3. Methodological constraints with the technique of forearm plethysmography have to date precluded an individual beat-by-beat examination of this response, in particular for resolving the question of the immediate flow pattern after release of venous occlusion. It has been suggested by Caro, Foley & Sudlow [Journal of Physiology (London) (1970), 207, 257–269] that there is a delay of up to five systolic beats before vasodilatation takes place, leading to their suggestion that the vasodilatation is passive and secondary to an increased flow through emptied capacitance vessels.
4. The introduction of peripheral Doppler techniques has led us to re-examine this response in an attempt to define short-term resistance vessel behaviour on a beat-by-beat basis.
5. Our data confirmed the hypothesis of Caro, Foley & Sudlow [Journal of Physiology (London) (1970), 207, 257–269] that there is a constant and definite latency preceding the onset of vasodilatory flow, as reflected by changes in Doppler velocities.