Circulation system for the coolant in an oil cooler (13) adapted to cool down the oil of the automatic transmission (2) of a motor vehicle in which the drain conduit (15) for the coolant from the oil cooler (13) of the transmission (2) is connected to said return conduit from the radiator, and that the exit mouth (16), of said drain conduit is inserted in the return conduit forming an angle A, said angle being more than 0° and less than 90°.