Abstract: A control valve is provided for use in a hydraulic steering system of an agricultural vehicle of the type having a steering valve of the reactive type for controlling the supply of hydraulic fluid to hydraulic cylinders having opposed working chambers serving to move the steerable vehicle wheels in opposite directions. The control valve is connectable between the steering valve and the hydraulic cylinders and has a first setting in which fluid flow in both directions between the steering valve and the working chambers of the hydraulic steering cylinders is possible at all times and a second setting in which return flow of hydraulic fluid from any one working chamber of the hydraulic cylinders to the steering valve is only possible when a positive pressure is present at the outlet of the steering valve connected by the control valve to the opposed working chamber of the hydraulic cylinders.