A program is being conducted to determine the mechanical behavior of irradiated Zircaloy-4 fuel rod cladding subjected to anneals under temperature and time conditions that simulate various off-normal and transient reactor operating conditions. This paper reports the initial phase of the program, which is a study of changes in hardness and tensile properties that accompany isothermal and transient anneals. A special transient annealing apparatus was built to subject specimens to the desired temperature/time conditions. Hardness and tensile strength decreased with increase in the maximum annealing temperature from 800°F (425°C) to 1300°F (705°C). Changes above 1100°F (595°C) were related to recrystallization and grain growth.