Whether a reactor pressure tube failure is ductile or brittle could spell the difference between an annoying incident and one producing ruinous mechanical damage to the plant. For this reason, it was imperative that a test be provided to assess directly the resistance to brittle failure of Zircaloy-2 pressure tubes. Theories of brittle fracture as applied to tubular configurations do not appear to be advanced enough to allow confident prediction of behavior based on indirect tests such as Charpy impact, drop weight, and others. Two methods were developed for introducing a starting crack in specimens of pressure tubing under controlled conditions of temperature and stress. Using these methods, data have been generated defining the effects of temperature, degree of cold-work, flaw size, hydrogen content, and a moderate amount of irradiation on susceptibility to brittle fracture. The tests indicate that at elevated temperatures likely to be encountered in reactor operation there exists no tendency toward brittle fracture arising from defects of the order of 1/2 in. long. At 150 C and below susceptibility to brittle failure increases with degree of cold-work. Since irradiation and cold-work have similar effects on strength and ductility it seems reasonable to expect brittle fracture to occur more readily with increasing exposure to reactor environment. Hydrogen content lowers the resistance to brittle failure at room temperature of a tube containing a flaw and increases the sensitivity of the test to flaw size. Continuing tests will explore the effects of increased flaw size and hydrogen content, elevated temperatures, and the effects of further irradiation.