This paper presents results from dynamic calorespirometric measurements on the two mould fungi Penicillium roqueforti and P. camemberti growing on agar. The measurements were made with two isothermal heat conduction calorimeters connected by a tube. In one of the calorimeters, the sample was placed and the other contained a carbon dioxide absorbent. Pressure sensors were connected to both the ampoules. The equipment also contained a valve on the tube that was opened and closed at regular intervals. Measurements were started at normal atmospheric pressure and gas composition, and continued after oxygen was consumed. The response of the fungi to the changing gas composition was followed and gas exchange ratios and metabolic enthalpies were calculated by approximate methods.