The method for thermally conditioning preforms in accordance with the invention comprises the following steps:—Placing the preforms on a chain conveyor (2);—Conveying the preforms through a first thermal conditioning stage (1) and subjecting them to a first heating or cooling heat exchange;—Removing the preforms from the first thermal conditioning stage and holding them outside for a predetermined amount of time in order to redistribute the temperature inside the preforms;—Subjecting the preforms to at least a second heating or cooling heat exchange in a second thermal conditioning stage (1) or in the first thermal conditioning stage. The invention also concerns the plants for heating or cooling preforms in accordance with the above method.