A pressure regulation servo-valve comprising a body having a utilization port, a feed port, and a return port, a spool mounted as a sliding fit in the body, the spool co-operating with the body to define a pilot chamber connected to the utilization port. The spool and the body together further define a priming chamber connected to the feed port via a second constriction and connected to the nozzle via a third constriction, and in which there exists a priming pressure acting on the spool in the first direction.