An automatic adjustment valve (20), designed to maintain at a constant level the differential pressure in a heating or cooling system (1), is connected into a return line (16) between radiators or the like (8, 9) and a main return pipe (4). The valve is via a measuring pipe (22) connected to a feed line (6) between a feed pipe (2) and said radiators or the like. In a valve body (23) and a top piece (26) of the valve, connected to the valve body a spindle (27) is mounted having a valve cone (25) designed to interact with a seat (24) in said return line (16). According to the invention, the spindle (27) is surrounded by partly a compression spring (43), which is supported by a stop element (45), which is threaded onto the spindle and is axially displaceable but not rotatable, and partly a membrane (51), which via a membrane disc (50) actuates the spindle in its closing direction against the action of said spring. The spindle extends through the membrane and is adjustable in this area in a predetermined way in operation by means of a tool (58), which can be inserted from outside, designed to rotate the spindle in relation to said stop element.