Abstract: Method and apparatus for determining in a person's exhalation air the concentration of a specific substance in the blood by measuring the concentration of said substance and the concentration of water vapor in the exhalation air and utilizing a known relationship between these concentrations. When the method is applied the exhalation air is exhaled freely in a defined air volume having a predetermined composition, and said concentrations are measured in this air volume. The apparatus for working the method comprises a device (16) which defines a space for receiving the exhalation air, which has two mutually opposite openings through which the space communicates with the surrounding air, and means (18, 22, 23) for selective quantitative detection of said substance in the air in the defined space.