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The effect of root-canal irrigation with different concentrations of sodium hypochlorite (3%, 5.1%, 7.3% NaOCl) on the mechanical properties of teeth was investigated in vitro. Root canals of 13 extracted, human premolars, denuded of enamel, were prepared with nickel-titanium rotary instruments (Quantec™) to a standard size by using saline irrigation. An electrical strain gauge was bonded to the cervical aspect of each tooth. The 10 experimental teeth were subjected to 5 successive, 30-minute periods of irrigation. The irrigants were used in the following order: (a) saline; (b) 3.0% NaOCl; (c) 5.1% NaOCl; (d) 7.3% NaOCl; (e) saline. Three control teeth were irrigated with saline only for all five periods. After each irrigation, the teeth were cyclically loaded to 110N while the surface strain was measured. Changes in strain of the test teeth after each irrigation regimen followed broadly similar patterns that were different from the control teeth. There was no difference, however, in the strain recorded after irrigation by the different irrigants within the experimental group.