An actuator is described with a prestressed reduction gear, which acts via a drive organ, particularly in the form of a drive segment, of a linear drive or a drive spindle upon an actuator for regulating a gas or fluid flow, particularly in the fields of heating ventilation, air-conditioning and fire and smoke protection. Actuating movement is limited by end stops. At least one inner end stop on the housing side limits the rotational freedom of an end wheel, which is in a positive fit with the drive organ, up to more than a full rotation. In a prestressed reduction gear, a switching device g can comprise two spur gears which have identical diameter and can be driven in the opposite direction for changing the M direction of movement of the drive organ, each of which have a pinion which can be displaced in the axial direction. The pinions are in each axial position in a positive fit with the drive organ. One of the two pinions, respectively, can be locked in a torque fixed manner with one of the two pinions having the same diameter.