A hydraulic shock absorber having an inner housing portion slidably coupled to an outer housing portion to define a variable size chamber for containing shock absorber fluid. The region where the inner and outer housing portions overlap defines an annulus between adjacent surfaces of the inner and outer housing portions which varies in size in accordance with the extension state of the shock absorber. A dynamic seal is coupled to a surface of the shock absorber within the annulus for confining shock absorber fluid to the chamber. The shock absorber fluid is an electro-rheological or magneto-rheological fluid and the shock absorber includes a device for generating a magnetic or electric control field within the chamber at a region adjacent dynamic seal in order to increase the viscosity of the shock absorber fluid adjacent to the dynamic seal to inhibit passage of the shock absorber fluid beyond the dynamic seal.