Abstract: A process for the setting of a constant temperature for a fluid, and furtmore, to a thermostat for implementing the process. The constant temperature is set by the heating up of the fluid during its flow through a heat source. The fluid, prior to the setting of the constant temperature, is initially cooled down to a lower temperature, such that the temperature difference which is reached between the constant temperature and a lower temperature, is higher than the temperature-overshooting range of the cooling regulating circuit. These measures simplify the setting of the constant temperature inasmuch as only the heat source need be equipped with a high-performance regulator. Through the cooling section which is connected ahead of the heating element, there is maintained a minimum temperature differential.