Theoretical consideration of the factors involved in maintaining vascular equilibrium and experimental evidence in animals and man indicate that at high tissue pressures there may be a sharp decline in blood flow and, in certain situations, complete cessation of blood flow even at tissue pressures below mean arterial pressure. At least two mechanisms appear to be involved: active closure of small arterioles under vasomotor tone when transmural pressure is lowered, either by falls in intravascular pressure or rises in tissue pressure, and passive collapse of soft-walled capillaries when tissue pressure rises above intracapillary pressure. These mechanisms are likely to assume particular importance when tissues are surrounded by non-compliant fascia and may thus be involved in compartmental syndromes.