Abstract: A shock absorber having a piston movable against a head of fluid under externally applied loads transmitted through a hollow piston rod. The piston has a valve surrounding a hollow metering pin defining a primary orifice and an auxiliary orifice, adjustable in size, is formed in the housing for augmenting the primary orifice area. The hollow metering pin provides a passage establishing communication between the auxiliary orifice and an accumulating reservoir located within the hollow piston rod. A fluid circulating system is provided for returning displaced fluid and for dissipating the heat generated in such fluid.