Abstract: An on-off valve is provided for a rockdrill which is operable by a hydraulic fluid under pressure and which is controlled by a hydraulically biassed valve. The rockdrill is turned on and off by opening and closing the on-off valve. When the valve is turned off hydraulic pressure from an inlet line and a supply pin line to a valve line of the hydraulically biassed control valve is cut off, thereby eliminating the pressure on the supply pin. This interrupts the operating cycle of the control valve and thereby shuts off the operation of the hydraulic drill. When the on-off valve is opened the hydraulic pressure from the inlet line via the supply pin line is re-established to the valve line and to the supply pin, and the operating cycle of the control valve is re-established.