A gas perfusion microcalorimetric system has been developed for metabolic investigations of plant tissues under dark conditions, The system consists of two twin heat conduction microcalorimeters connected by gas-tight tubing. During measurements, a flow of humid gas enters the main calorimeter containing the plant tissue. The volume of this sample chamber can be varied between 4 and 25 ml and the flow rate of the gas can be varied up to maximum of 150 ml h(-1). The outgoing air is allowed to pass through a second calorimeter, serving as an on-line CO2 analyzer. Gases, such as NOx or CO2, can be added to the incoming gas-flow either continuously or by stepwise injection. The calorimetric system has been tested in respiratory experiments with potato tuber slices.