Device for the mixing and distribution of fluids for a catalytic reactor with a downward flow, said device comprising at least one collection zone (A), at least one mixing zone (B) comprising at least one enclosure (15) for the mixing of the fluids, at least one distribution zone (C), characterized in that said mixing zone (B) is situated at the same level as the distribution zone (C) and also comprises at least one enclosure (16) for the exchange of the fluids, connected to, and communicating with, said mixing enclosure (15), said exchange enclosure (16) comprising at least one lateral passage section (17a, 17b) suitable for the passage of the fluids from said exchange enclosure (16) to said distribution zone (C).