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 (11, 12); 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 (11, 12) 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 (11, 12); 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 (11, 12) 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.