Nuclear research and power reactors are now being designed which must operate at temperatures and coolant environments dictating the use of nickel-base and refractory metal alloys for structural components, fuel cladding, and power equipment. Investigation of the effects of irradiation and environment upon the mechanical properties of several nickel-base and refractory metal alloys has been initiated at the Pacific Northwest Laboratory. Tension specimens of Inconel 600, Inconel 625, Inconel 702, Inconel 718, Incoloy 800, Hastelloy C, Hastelloy N, Hastelloy X-280, and Hastelloy R-235 have been irradiated to exposures exceeding 1 x 1021 nvt (E > 1 Mev) in 50 C water, 280 C water, and 740 C gaseous environments. These specimens have been tested in the pre- and postirradiated conditions at room temperature, 300 C, and 650 C. Several metallurgical conditions of the materials have also been examined and the changes in mechanical properties due to the irradiation have been noted. The Cb-1 Zr, Cb-752, TZM Mo, and W-25 Re refractory metals have been irradiated at ambient temperatures, and the effects of irradiation upon the mechanical properties have been evaluated at room temperature and 1090 C. The data obtained for both the nickel-base and refractory metal alloys tested indicate that there is a marked effect upon the mechanical properties due to the thermal environment. In addition to this effect, effects due to the irradiation were also found. Comparison of the elevated temperature data to the room temperature data shows a marked decrease in both strength and ductility. Metallographic examination of these materials is continuing in an attempt to find the cause of these damages.