Recently developed water and strain-gauge plethysmographs are compared on the same forearm. They do not differ systematically in absolute blood-flow values at different steady-state levels, provided that the strain gauge is applied at the middle, muscular, part of the forearm. The accuracy of the water and strain-gauge plethysmograph is identical, and the two instruments closely follow random variations in blood flow. The water plethysmograph is a reliable integrator of blood flow in a segment of a limb, and the strain-gauge described here can be used as an adequate substitute, when placed in the middle of the limb segment examined. However, the results from the strain-gauge plethysmograph are dependent on the actual location of the strain gauge.