The effect of loading rates, ranging from 10 to 100,000 ksi square root of (in/sec), and temperatures, ranging from room temperature to -100F, on the plane strain fracture toughness of several high strength alloy steels has been determined. Materials investigated are a commercial 4340 steel, a modified 4330 steel from a gun tube forging having three different heat treatments and two different heats of 250 grade maraging steel. Test specimens utilized are essentially ASTM 'compact tension' type specimens of one inch thickness. Tests were conducted on an open loop, hydraulic, high loading rate tensile testing machine. Results are presented in the form of graphs of fracture toughness versus temperature for the maximum and minimum loading rates ('dynamic' and 'static'). Fracture toughness versus loading rate at -60F and yield strength versus elastic strain rate at room temperature, -60F and -100F for one heat of maraging steel are also reported.