Steel samples of reactor pressure vessel and piping steels from the German HDR programme have been tested in high oxygen water at different temperatures simulating HDR test conditions. The specimens have been exposed to sequences of static and cyclic loading or to purely cyclic loading. During the tests, threshold stress intensity values for stress corrosion cracking and crack growth rates with various cyclic loading parameters were determined. Extensive fracture surface and oxide layer investigations were also performed. Water chemistry parameters such as dissolved oxygen content, pH and conductivity were continuously monitored during the tests. Finally, the measured laboratory water chemistry parameters were compared to those measured in the HDR plant during full scale testing of components and the relevance of the results for normally operating plants is discussed.