Abstract: An actuator head is provided for a pressure relief valve (1) having a valve element (11) spring biased to a closed position and arranged to be movable under the action of a fluid pressure medium in order to allow the escape of the fluid pressure medium through the valve for pressure relief purposes. The actuator head comprises an actuator (3) which is coupled with the valve element (11) and has a member (21) displaceable in response to fluid pressure applied thereto. A linkage (22, 19, 12) couples together the valve element (11) and the actuator (3) and has a lost-motion connection (72) which allows the valve element (11) to move against its spring biasing (13) for pressure relief purposes without imparting movement to the displaceable member (21) of the actuator (3). Biasing means (27) is arranged to oppose displacement of the displaceable member (21) of the actuator (3), and control means (4, 5) is provided to control the supply of fluid pressure to said displaceable member (21).