Drop down seal for doors or windows with a sliding panel, comprising: - a channel-shaped external support profile suitable to be fixed to the base of the door or window panel; - a rod-shaped support element; - a movable element connected to the support element by means of a parallelogram articulation mechanism suitable to allow the movable element to move in height between a raised rest position and a lowered working position, - at least one sealing gasket carried by the movable element and suitable to rest on an abutment surface when said movable element is in the lowered working position; wherein the drop down seal is characterized in that said movable element, when in the rest position, is also operationally and removably connected to the support profile; and in that the rod-shaped support element is slidably housed in the support profile; and in that the support element, at one of the proximal ends thereof, forms, or is provided with, a laterally protruding control portion suitable to be engaged by a trigger base fixed to the abutment surface in such a way that, when the panel is moved from the open position to the closed position along a movement axis of the panel, just before reaching the closed position, the movement of the support element is stopped by the trigger base and the further movement of the support profile causes the movable element to be lowered. - a channel-shaped external support profile suitable to be fixed to the base of the door or window panel; - a rod-shaped support element; - a movable element connected to the support element by means of a parallelogram articulation mechanism suitable to allow the movable element to move in height between a raised rest position and a lowered working position, - at least one sealing gasket carried by the movable element and suitable to rest on an abutment surface when said movable element is in the lowered working position; wherein the drop down seal is characterized in that said movable element, when in the rest position, is also operationally and removably connected to the support profile; and in that the rod-shaped support element is slidably housed in the support profile; and in that the support element, at one of the proximal ends thereof, forms, or is provided with, a laterally protruding control portion suitable to be engaged by a trigger base fixed to the abutment surface in such a way that, when the panel is moved from the open position to the closed position along a movement axis of the panel, just before reaching the closed position, the movement of the support element is stopped by the trigger base and the further movement of the support profile causes the movable element to be lowered. wherein the drop down seal is characterized in that said movable element, when in the rest position, is also operationally and removably connected to the support profile; and in that the rod-shaped support element is slidably housed in the support profile; and in that the support element, at one of the proximal ends thereof, forms, or is provided with, a laterally protruding control portion suitable to be engaged by a trigger base fixed to the abutment surface in such a way that, when the panel is moved from the open position to the closed position along a movement axis of the panel, just before reaching the closed position, the movement of the support element is stopped by the trigger base and the further movement of the support profile causes the movable element to be lowered.