The invention relates to a method for measuring the concentration of substances in living organisms using microdialysis, whereby bodily fluid is mixed with a perfusate and passed through a membrane and a substance value is determined for the bodily fluid with the aid of a sensor unit. According to the invention, to allow for the limited transport through the membrane, so that the measured substance value differs from the actual substance value in the organism, a reference substance present in said organism with a known value is also measured after passing through the membrane and mixing with the perfusate and the actual substance value is determined, based on the quotient from the known reference substance value divided by the measured reference substance value, multiplied by the measured substance value.