Abstract: A method of checking the dimensions of the extensions of control rods of a nuclear reactor when the extensions are located in a cooling pit or fuel storage pool under a safe depth of water, comprising placing each extension vertically on a reference support in the cooling pit, the support having a profile corresponding to that of the extensions, moving measuring pick-ups, calibrated by reference to the support, along the extension, measuring the distances between the pick-ups and the extension at a plurality of levels, and analyzing the measurements. An apparatus is also disclosed for performing the novel method of checking rod-extension dimensions in the nuclear reactor.