The paper reports on a monitoring program comprising mechanical testing of Nimonic PE16 alloy tie bars after irradiation in the civil advanced gas-cooled reactors. The high-temperature stress rupture properties and the strength at room temperature along the bar were determined with increasing irradiation. It was shown that these properties varied markedly with axial location on the bar as a result of the imposed fast and thermal neutron fluxes and the temperature profile existing in the reactor. Also investigated was the sensitivity to stress of the high-temperature rupture properties and the effect of annealing periods on the room temperature proof strength.