The forces produced by 10 orthodontic archwires were measured in a simulated clinical situation and also with the wires in simple three point loading. All tests were made to a maximum deflection of 3 mm. At 1·5 mm deflection the forces exerted when each wire formed part of a fixed appliance ranged from 1·5 to 8·3 N. When the same wires were used as simple beams the forces exerted ranged from 0·3 to 3·0 N. The findings indicate that the forces generated by orthodontic mechanisms cannot be calculated from straight forward physical principles.