Abstract: A chain saw for cutting slots in a wellbore casing and the surrounding formation comprises a pair of cutting chain assemblies (37) rotatably mounted on a body and moveable from a storage position in which the axis of the cutting chain assemblies are parallel to the axis of the tool to a position at right angles to the storage position. The chains are driven by a mud-motor, step-up gearbox, drive shaft (20) and bevel gear (34). The position of the chain arms is controlled by a mud operated piston (44) which is connected by rods (41A) to racks (41) which mesh with pinions provided on the arms of the chain assemblies (37). Pressurization of the drillstring upon which the tool is mounted will cause rotation of the motor to drive the cutting chains and simultaneous downward displacement of the piston (44) to bring the cutting chains into engagement with the casing.