A metering unit for agricultural seeding and and pneumatic fertilizing machines, comprising an external structure (10) shaped like a hopper, inside which there is a metering spool (11), which is substantially cylindrical and rotatable by means of an external transmission (12). The metering spool (11) is configured to deliver a prefixed quantity of product, such as seeds and/or fertilizer, which is mixed with an air flow and sent to one or more distribution groups of the agricultural machine. Moreover, the metering spool (11) has an axis of rotation placed in a horizontal direction with respect to a movable bulkhead (16； 26)； the movable bulkhead (16； 26) is arranged below the metering spool (11) and placed in a transverse direction with respect to the rotation axis of the metering spool (11).
