Abstract: A method and apparatus for determining the carbon dioxide content of micro samples of blood, serum, or other body fluid. In performing the method, a micro quantity of sample is introduced into one compartment of a double-compartmented open-topped reaction vessel, the other compartment containing a reagent capable of liberating the carbon dioxide from the sample when the two are mixed. The open top of the vessel is then sealed so that its interior communicates only with a capillary gasometer, and the contents of the vessel are then thoroughly mixed. The volume of evolved gas is measured directly from the graduated capillary gasometer by observing the extent of displacement of an indicating liquid therein. The structure of the device is described, along with the structure and method for adjusting the volume of the reaction vessel, after sealing and before mixing, to locate the meniscus of the liquid in the capillary at a selected starting point.