1. The blood pressure inside the leg of the house spider Tegenaria atrica has been measured. Maintained pressures of about 5 cm. Hg and transient pressures of up to 40 cm. Hg have been found.
2. The relation between the blood pressure in the leg and the extension torque at the hinge joints has been established.
3. Considerable torques can be developed at the hinge joints during extension, for example, when accelerating a mass fixed to the leg. The transient pressures found to arise in the leg are adequate to account for these torques.
4. The hydraulic mechanism is discussed. The available evidence suggests that the pressure found in the legs occurs also in the prosoma but not in the abdomen, in which case the maintained pressure must be due to the heart. This, however, requires further investigation