AbstractObservations were made on the circulatory effects of an occlusion cuff applied distally to a forearm segmental plethysmograph. Measurements of the forearm blood flow (plethysmographically and with thermal conductivity recorders) and the pressure in the brachial artery, were performed. The plethysmographic registration of blood flow (arterial inflow) in a forearm segment necessitates a temporary blockade of the more peripheral venous return. The commonly used arterial occlusion was shown to be only partially adequate; besides other disadvantages, it was regularly accompanied during the first 1–2 min by a gradual increase of the blood flow in forearm muscle and skin, and there was also an immediate rise of the local arterial blood pressure. The distal occlusion cuff had to be kept in a constant position as a displacement in the distal direction of only 1–2 cm frequently gave rise to a considerable increase of the plethysmographically measured blood flow values, especially when the forearm or hand circulation was high. A cuff position directly distal to the plethysmograph should be avoided as it causes proximal tissue displacement and probably vessel compression. Every mode of application of the distal occlusion cuff is a compromise between different disadvantages. For venous occlusion plethysmography this means that it is impossible to measure accurately the true blood flow (arterial inflow) in limb segments. On forearm the optimal results were frequently obtained with the cuff 1–2 cm distal to the plethysmograph, and with simultaneous inflation of the distal occlusion and proximal collecting cuff to the same venous occlusion pressure (i. e. about 50–80 mm Hg under normal cond