The invention relates to a method for regulating rotational speeds of compressors which are arranged in series, for compressing a fluid, in particular cryogenic helium, wherein a setpoint inlet pressure which the fluid is supposed to be at at an inlet of the compressor which is arranged furthest upstream is predefined, an actual inlet pressure of the fluid at that inlet is detected, an actual outlet pressure of the fluid at an outlet of the compressor which is arranged furthest downstream is detected, an actual overall pressure ratio is detected, wherein the actual overall pressure ratio corresponds to the quotient of the actual outlet pressure and the actual inlet pressure, a proportional integral value is determined from the deviation of the actual inlet pressure from the setpoint inlet pressure, a capacity factor is determined from the proportional integral value and the actual overall pressure ratio, a model overall pressure ratio is determined from the actual overall pressure ratio and the capacity factor, a reduced setpoint rotational speed is determined for each compressor, wherein the respective reduced setpoint rotational speed is determined as a functional value of a regulating function which is assigned to the respective compressor and assigns a reduced setpoint rotational speed to each value pair comprising capacity factor and model overall pressure ratio, and the rotational speed of each compressor is set using the reduced setpoint rotational speed which is determined for it.