A mechanical valve for fluids, comprising: - a valve body provided with an inlet port , an outlet port and an inner passageway for communicating said inlet and outlet ports ; and - a stem provided with an end housed in the inner passageway of the valve body with the capacity of rotation between: an opening position, in which fluid passes between the inlet and outlet ports of the body , and a closed position, in which said fluid passage is prevented; wherein - the inner passageway of the valve body is cylindrical and - the stem comprises a cylindrical tubular segment , tightly housed, with the capacity of rotation and without the capacity of axial displacement in the inner passageway of the valve body , and an opposite end protruding from the body for rotational actuation; said tubular segment comprising: - an inner cavity for fluid circulation in an opening position of the valve, - radial windows , - radial fluid outlet ports (25a - 25e), of different sections, circumferentially distributed in the tubular segment and arranged in correspondence with different opening positions of the stem , forming an alignment of radial ports (25a- 25e) of increasing section, suitable for facing the outlet port of the valve body in successive angular positions of the stem and providing different fluid flows and - a closing portion , facing the outlet port of the body in a determined angular position of the stem , closing the fluid passage, characterised in that said radial windows permanently communicate the inner cavity of the stem with the inlet port of the valve body . - a valve body provided with an inlet port , an outlet port and an inner passageway for communicating said inlet and outlet ports ; and - a stem provided with an end housed in the inner passageway of the valve body with the capacity of rotation between: an opening position, in which fluid passes between the inlet and outlet ports of the body , and a closed position, in which said fluid passage is prevented; wherein - the inner passageway of the valve body is cylindrical and - the stem comprises a cylindrical tubular segment , tightly housed, with the capacity of rotation and without the capacity of axial displacement in the inner passageway of the valve body , and an opposite end protruding from the body for rotational actuation; said tubular segment comprising: - an inner cavity for fluid circulation in an opening position of the valve, - radial windows , - radial fluid outlet ports (25a - 25e), of different sections, circumferentially distributed in the tubular segment and arranged in correspondence with different opening positions of the stem , forming an alignment of radial ports (25a- 25e) of increasing section, suitable for facing the outlet port of the valve body in successive angular positions of the stem and providing different fluid flows and - a closing portion , facing the outlet port of the body in a determined angular position of the stem , closing the fluid passage, characterised in that said radial windows permanently communicate the inner cavity of the stem with the inlet port of the valve body . - an inner cavity for fluid circulation in an opening position of the valve, - radial windows , - radial fluid outlet ports (25a - 25e), of different sections, circumferentially distributed in the tubular segment and arranged in correspondence with different opening positions of the stem , forming an alignment of radial ports (25a- 25e) of increasing section, suitable for facing the outlet port of the valve body in successive angular positions of the stem and providing different fluid flows and - a closing portion , facing the outlet port of the body in a determined angular position of the stem , closing the fluid passage, characterised in that said radial windows permanently communicate the inner cavity of the stem with the inlet port of the valve body . <EOS>