A heat exchanger is disclosed which has a collecting pipe having formed therein at least one slot whereby the slot lies in a plane roughly perpendicular to the collecting pipe longitudinal axis. A flat pipe, which is capable of accommodating a fluid, a partition, or the like can be inserted into the slot, the narrow sides of the slot being limited by parallel-running slot walls of the collecting pipe which extend roughly along a tangent to a circle around the pipe longitudinal axis. The slot length, determined by the spacing between the slot walls, corresponds at least to the pipe inner diameter. A method of making a slot in a collecting pipe also is disclosed whereby in order to produce the smallest possible pipe diameter and high compressive strength, a stable and seal-tight connection of a laterally issuing structural part is ensured. To prepare such a slot, it is envisaged that the slot walls are formed by deformation of the material of the collecting pipe, which slot walls have a constant wall thickness over their wall length, the outer sides of the slot walls lying for the most part outside the pipe outer casing. A heat exchanger having a slot formed in a, collecting pipe, such as a condenser, a heating element, an oil cooler, a charge cooler or the like is disclosed.