A speed governor is utilized in an air driven turbine dental handpiece having both an air inlet passage to said turbine and an air release passage from said turbine. A valve is included in the inlet passage and is moved in response to the velocity or to the pressure of air from the turbine in the outlet passage. The valve includes a valve body movable between different air flow positions in response either to a spring restrained vane or to a pressure responsive piston or diaphragm, the net result being to maintain the turbine velocity reasonably constant under varying load and at or below a predetermined maximum.