We have tested the mechanical properties of isolated canine pericardium at 37°C in Hanks' solution, and compared those results with similar tests on tissue at room temperature, both in Hanks' solution and merely kept moist with saline. Pericardium, which is nearly isotropic due to the layers of collagen aligned in different directions, is a composite material made up of collagen and elastin fibers in a viscous ground substance matrix. Rapidly applied loads result in a stress-strain response similar to the previous in vivo pressure-volume curves. Slowly applied loads produce an accommodatioii effect as fiber geometry rearranges through the viscous ground substance. Plasticity of the pericardium, long accepted as fact, is not present An accommodation effect is produced instead by shifts in the stress-strain curve due to cyclic loading, stress relaxation, and creep. The slow time course of the accommodation effect is responsible for the differences in hemodynamic effect between an acute and a chronic pericardial effusion. The mechanical properties of the pericardium are due primarily to collagen, which establishes the high ultimate tensile strength and high final slope of the stress-strain curves. The initial extensible portion of the stress-strain curve probably is due to initial rearrangement of collagen fiber weave under stress. Temperature dependence and previous digestion studies indicate a major role for elastin in determining the stress-strain response and limits to stress relaxation and creep. Thixotropy of the ground substance matrix may produce rate Independence In rapid filling of the pericardial sac Circ Res 49: 1981