An arrangement for automatic drift compensation in a gas analyzer in which a source of radiation radiates through the gas to be analyzed, and directs the radiation onto a detector. The latter forms an amplified output signal corresponding to the detector output and corresponding to the gas concentration. A signal storage stores the signal corresponding to the output signal of the detector when the gas to be analyzed has a predetermined concentration. A comparator compares the output signal from the detector with the stored signal. The gas analyzer is used for determining the content of a predetermined gas in a mixture of gases exhaled by a patient. A circuit connected between the detector output and the comparator input delivers to the storage a signal during the inspiration phase of the patient.