The capillary transport from tissue to blood of iodide, expressed as the capillary diffusion capacity or permeability-surface area product (PS), has been investigated in vasodilated dog skeletal muscle in 3 shock models: hemorrhagic shock, intestinal stasis with hemoconcentration, and RBC aggregation induced by HMWD. The dependence of PS on perfusion pressure and flow was investigated and compared with PS at acute reduction of flow and perfusion pressure. In shock, there was a 40 to 60-percent reduction of PS at perfusion pressures above 60 mm Hg. There was a further reduction at lower pressures, so that at 40–50 mm Hg, PS was 5–25% of control at normal perfusion pressure. With acute reduction of perfusion pressure to 40–50 mm Hg, PS fell only to 60–70% of control. The mechanism behind these 2 phases of reduction of diffusional transport may be obstruction of capillaries and inhomogeneous distribution of flow. Some preliminary results on the treatment of the transport disturbance were presented, and it was shown that the disturbance was easily reversed by LMWD and more sluggishly by fresh autologous blood, while ‘old’ blood was inefficient within the same time. The possible importance of the gain in transport by this type of treatment in a resistive hypotensive state is commented upon.