A device for periodically supplying measured quantities of a pressure fluid to a nozzle adapted to blow a weft thread into the shed of a pneumatic loom, comprising a metering chamber fed by a pressure fluid source and connected to the nozzle through a periodically opening valve means, control means being provided to influence the behaviour of the weft thread and more particularly to stabilize the position of the weft thread in the fluid jet irrespective of the nature of that thread.