A tool control mechanism is configured to activate and deactivate a drill string tool by hydraulic action of drilling fluid. The tool control mechanism is switchable between an activation mode and a deactivation mode. In the activation mode, a hydraulic activator ram is coupled to a tool switch member to drive the switch member in an activation direction in response to above-threshold drilling fluid conditions. In the deactivation mode, a deactivator ram is coupled to the tool switch member to drive the switch member in a deactivation direction opposite to the activation direction, when above-threshold drilling fluid conditions occur. The tool control mechanism is switchable between the activation mode and the deactivation mode by operator-controlled drilling fluid pressure variations.