A pressure activated device for controlling the supply of a gas and a breathing system for underwater use incorporating the device are provided. The device comprises an input port (64) for connection to a pressurised gas supply, an output port (66), a chamber (62) and a pressure monitoring port (66). Flow control means are provided for selectively opening a fluid path (65) outside the chamber between the input port and the output port when the ambient pressure is higher than the pressure in the chamber by a more predetermined amount. Reset means (80) selectively open a fluid path (84, 92) between the pressure monitoring port and the chamber, when the pressure at the pressure monitoring port is higher than the ambient pressure by more than a predetermined amount. The device may be used to control the supply of diluent gas to an underwater breathing system, or to maintain the volume of a flexible enclosure substantially constant, irrespective of variations in the ambient pressure.