The possible role of the physical and mechanical properties of the neural arch as contributing factors in the etiology of spondylolysis was tested by comparing two groups of vertebral specimens obtained at necropsy. Group I specimens had been subjected to repetitive cyclic force which they withstood without fracturing. Group II specimens were randomly selected. The cross-sectional distribution of cortical and cancellous bone in the partes interarticulares were measured, and the two groups compared. The previously stressed group was found to have a greater percentage of cortical bone than the random group. The data are interpreted to suggest that thin partes interarticulares may predispose to spondylolysis through the mechanism of fatigue failure.