The bending stiffness of superelastic nickel-titanium archwires is influenced by alterations in mouth temperature. The activation and deactivation phases of a load-deflection loop of superelastic wires have different stress-magnitudes. This investigation compared the effect of short-term cooling or heating on the bending force exerted by nickel-titanium archwires. Two rectangular superelastic and one conventional nickel titanium wire were tested in bending at 37 degrees C. The specimens were tested during the activation phase and during the deactivation phase. The wires were kept at constant strain and the bending force was measured continually while the activated specimens were subjected to cold (10 degrees C) or hot (80 degrees C) water. The test situation simulates a patient's archwire that is subjected to cold or hot drinks or food during a meal. The conventional nickel-titanium wire was marginally affected by brief cooling or heating, regardless of activation phase. In contrast, the superelastic wires were strongly affected by short-term application of cold or hot water. When tested in activation phase, the effect of heating was transient whereas the wires continued to exert sub-baseline bending forces after short-term application of cold water. When tested in deactivation phase, the effect of cooling was transient whereas the wires exerted supra-baseline bending forces after a short-term application of hot water. The effect of short-term temperature changes on the bending stiffness of superelastic nickel titanium archwires is dependent upon the bending phase. Cooling induced transient effects on a wire in its deactivation phase, but prolonged effects when the wire was tested in the activation phase. In contrast, the effect of short-term heating was transient when the wire was tested in the activation phase, but prolonged when the wire was tested in the deactivation phase.