Abstract: An arrangement for the non-invasive determination of the minute volume Q of the heart is arranged to determine the amount of a sufficiently soluble gas, e.g. carbon dioxide or nitrous oxide, present in the blood, prior to and subsequent to the passage of the blood through the heart. The expired and inspired gases are separated from one another with each breath taken. Transducers feed respective devices for determining the gas content of the respiration gas, and the magnitude of the expiration gas flow and the inspiration gas flow. These devices are controlled by a microprocessor, which is programmed to supply gas from a gas source, to meter the gas during a subsequent inspiration cycle, and to control an indicator for the Q-value. The gas is controlled in a manner such that the gas-content of the lungs is changed in accordance with a given program. The Q-value is calculated for pairs of respiration cycles in accordance with the formula: ##EQU1## in which V.sub.