The service life of a nuclear reactor fuel element is related to the ability of its metallic cladding to accommodate severe local plastic strains caused by fuel expansion or external coolant pressure. A technique has been developed for subjecting samples of thin-wall tubing to cyclic plastic strains similar to those experienced in reactor service. By applying gas pressure, the thin tubular specimen is alternately expanded and contracted between rigid concentric mandrels until failure occurs, as indicated by gas leakage. Relative cyclic strain range is determined by mandrel dimension, specimen diameter, and specimen wall thickness. At larger strain ranges, deformation occurs by reverse bending rather than simple tension and compression. These tests have been made at 1300 F on specimens of Hastelloy-X, Inconel-600, Incoloy-800, and Type 304 stainless steel; the last two in-reactor as well as in the laboratory. Specimen life in cycles-to-failure, N, was found to be related to strain range Δ ϵ0 computed from mandrel dimensions by NαΔ ϵ0 = C, in which α varied from 0.6 to 1.35 and C from 0.14 to 3.3 depending on material. Effective strain range was found to be less than Δ ϵ0 because of specimen wrinkling. The fatigue life of Incoloy-800 and Type 304 stainless steel was somewhat reduced by testing in fast neutron radiation. Specimen failure was by longitudinal cracking, with little evidence of gross plastic deformation, although there was evidence of deformation within the grains of the alloy in the vicinity of cracks.