A mechanically and hydrodynamically operated brewing unit (1) for automatic beverage vending machines includes a rotatable base (3) provided with a compartment (15) that acts as a brewing chamber, means to let water under pressure into said compartment, means to connect said compartment with the point at which the brewed beverage is dispensed, and a brewing head (4). This brewing head is comprised of a hollow containment structure (279), a first hollow piston (28) that is housed in said containment structure movably relative thereto, a second piston (29) that is accommodated in said first piston movably relative thereto, and elastic means (30) contrasting the movement of said second piston. The brewing unit is characterized in that said first piston and said second piston are operated by the direct action of a fluid under pressure.