Rotating-beam endurance tests at room temperature have been conducted on 0.86-in-diam minimum notch sections and 1.25-in-diam minimum unnotched sections in order to compare fatigue data obtained from geometrically similar specimens having minimum diameters of 0.19 in. and 0.30 in., respectively. Comparisons have been made for tension-test data obtained from sections of 0.19 in., 0.50 in., and 0.75 in. diam. A series of exploratory tests in fatigue were made on grooved specimens and for two notch angles for comparison with results obtained on ungrooved specimens. Results are reported for the optimum chemical composition without any nickel content in the annealed and as-cast condition.