A stacking unit of slats on a complete support ladder for the production of Venetian blinds, wherein the complete ladder comprises two uprights (P1, P2), which support a plurality of cross-members (T1, T2) of engagement with the slats arranged at regular reciprocal distances along the longitudinal extension of said uprights, wherein said stacking unit comprises: - means for guiding a slat along a longitudinal insertion lane on an insertion plane (m); - means (31, 32) for positioning a complete ladder on a positioning plane (p) incident to said insertion plane (m) at an engagement area with the slats, said positioning means comprising two U-shaped guides (31, 32) that are arranged, spaced transversely apart from each other, on two opposite sides of said longitudinal insertion lane and each of which defines a sliding seat for an upright (P1 or P2) of the ladder along a sliding direction (Y1, Y2) lying on said positioning plane (p), wherein each sliding seat is open on the longitudinal insertion lane by means of a slit which extends parallel to said sliding direction (Y1, Y2) to allow in use the passage of said cross-members (T1, T2) from one upright to another as well as the free sliding of said cross-members through the two U-shaped guides, at said slit each guide (31, 32) having a cross-section bottleneck that delimits said sliding seat; - means for associating the slat to said ladder at said engagement area by acting on said cross-members (T1, T2); and - a loader, arranged above the insertion plane (m), inside which, one at a time, the slats are progressively pushed, juxtaposed to each other, after having been associated to the ladder, characterized in that each of said U-shaped guides (31, 32) is divided into two parts (31', 31") reciprocally movable to pass between at least one closed guide position and at least one open guide position, wherein in the open guide position the cross-section bottleneck of said sliding seat defines a wider through opening than in the closed guide position. - means for guiding a slat along a longitudinal insertion lane on an insertion plane (m); - means (31, 32) for positioning a complete ladder on a positioning plane (p) incident to said insertion plane (m) at an engagement area with the slats, said positioning means comprising two U-shaped guides (31, 32) that are arranged, spaced transversely apart from each other, on two opposite sides of said longitudinal insertion lane and each of which defines a sliding seat for an upright (P1 or P2) of the ladder along a sliding direction (Y1, Y2) lying on said positioning plane (p), wherein each sliding seat is open on the longitudinal insertion lane by means of a slit which extends parallel to said sliding direction (Y1, Y2) to allow in use the passage of said cross-members (T1, T2) from one upright to another as well as the free sliding of said cross-members through the two U-shaped guides, at said slit each guide (31, 32) having a cross-section bottleneck that delimits said sliding seat; - means for associating the slat to said ladder at said engagement area by acting on said cross-members (T1, T2); and - a loader, arranged above the insertion plane (m), inside which, one at a time, the slats are progressively pushed, juxtaposed to each other, after having been associated to the ladder,