Use of a gas or gas precursor for the manufacture of a monitoring agent for a diganostic method for monitoring cardiac output in a mammal, including man, wherein said monitoring agent is a diagnostically acceptable gas in the gaseous state adapted for intravenous use, said gas being of such a nature, and being used in such an amount, that it is detectable via the expired breath from the mammal in question. An especially preferable gas is nitrous oxide. Monitoring agent for such a diagnostic method as well as such a diagnostic method.