Abstract: A regulating system for regulating the pressure in a fluid flow line particularly an L.P.G. supply line to the carburation system of an internal combustion engine comprises a regulating device connected in the flow line for varying the fluid flow rate along the line and an electro-fluid control loop comprising a feed-back line connected to the flow line to provide a feed-back pressure related to the pressure in the flow line and a control arrangement in which the feed-back pressure is compared with a reference pressure to provide an electrical output signal for controlling the regulating device in the sense to maintain a predetermined relationship between the feed-back pressure and the reference pressure.The control arrangement has a pair of control chambers, each control chamber being divided into sub-chambers by a flexible diaphragm and the feed-back pressure being connected to act in one sub-chamber and the reference pressure being connected to act in the other sub-chamber.