[en] The effect of thermal aging on the pressure vessel material at unit 4 of the Jaslovske Bohunice nuclear power plant was evaluated. Surveillance bodies were positioned above the reactor core where the flux of neutrons with E > 0.5 MeV was negligible. The operating temperature of the surveillance bodies was estimated to between 266 and 296 degC. No effect of thermal aging on the mechanical properties in static tensile stress of the base material, weld metal or the heat affected zone was observed at room temperature or at the pressure vessel operating temperature. The plastic properties improved for all of the materials. Thermal aging has virtually no influence on the notch toughness critical transition temperature. Notch toughness in the plastic range increased slightly for both the base material and the weld metal. (Z.S.). 28 tabs., 9 figs., 4 refs