A positive-displacement compressor with an automatic compression ratio-adjustment system (V i) adapted to be installed in a conditioning or refrigeration system for a working fluid, which compressor comprises: - a suction chamber for sucking the working fluid having a variable suction pressure (p a) in response to the system conditions; - a delivery chamber 4 for delivering the working fluid having a delivery pressure (p m) greater than said suction pressure (p a), varying in response to the system conditions; - a compression chamber interposed between said suction chamber and said delivery chamber and in fluid communication with the latter via one or more discharge ports; - at least one screw compression element accommodated in said compression chamber and rotating about a longitudinal axis to compress the working fluid to reach a compression pressure (p c); - motor means for driving said compression element; - adjustment means for adjusting the compression ratio (V i); wherein said adjustment means comprise mechanical valve means interposed between said compression chamber and said delivery chamber to automatically vary the aperture of said one or more discharge ports in response to the difference in pressure existing between said delivery chamber and said compression chamber; wherein said valve means comprise at least one movable element received in a corresponding seat, having an upstream portion in fluid communication with said compression chamber and a downstream portion in fluid communication with said delivery chamber; characterized in that said compression chamber has a substantially flat end wall, transverse to the axis of rotation of said at least one compression element, and defining a delivery plane, said at least one seat being adjacent to said delivery plane for the movement of said at least one movable element to instantaneously equalize said compression pressure (p c) with said delivery pressure (p m)to improve the efficiency of the compressor. - a suction chamber for sucking the working fluid having a variable suction pressure (p a) in response to the system conditions; - a delivery chamber 4 for delivering the working fluid having a delivery pressure (p m) greater than said suction pressure (p a), varying in response to the system conditions; - a compression chamber interposed between said suction chamber and said delivery chamber and in fluid communication with the latter via one or more discharge ports; - at least one screw compression element accommodated in said compression chamber and rotating about a longitudinal axis to compress the working fluid to reach a compression pressure (p c); - motor means for driving said compression element; - adjustment means for adjusting the compression ratio (V i); wherein said adjustment means comprise mechanical valve means interposed between said compression chamber and said delivery chamber to automatically vary the aperture of said one or more discharge ports in response to the difference in pressure existing between said delivery chamber and said compression chamber; wherein said valve means comprise at least one movable element received in a corresponding seat, having an upstream portion in fluid communication with said compression chamber and a downstream portion in fluid communication with said delivery chamber; characterized in that said compression chamber has a substantially flat end wall, transverse to the axis of rotation of said at least one compression element, and defining a delivery plane, said at least one seat being adjacent to said delivery plane for the movement of said at least one movable element to instantaneously equalize said compression pressure (p c) with said delivery pressure (p m)to improve the efficiency of the compressor. wherein said adjustment means comprise mechanical valve means interposed between said compression chamber and said delivery chamber to automatically vary the aperture of said one or more discharge ports in response to the difference in pressure existing between said delivery chamber and said compression chamber; wherein said valve means comprise at least one movable element received in a corresponding seat, having an upstream portion in fluid communication with said compression chamber and a downstream portion in fluid communication with said delivery chamber; characterized in that said compression chamber has a substantially flat end wall, transverse to the axis of rotation of said at least one compression element, and defining a delivery plane, said at least one seat being adjacent to said delivery plane for the movement of said at least one movable element to instantaneously equalize said compression pressure (p c) with said delivery pressure (p m)to improve the efficiency of the compressor.