A system has been developed for measuring non-shunt cardiac output by the throughflow technique, using nitrous oxide in patients undergoing general anaesthesia. The throughflow measurement technique is a non-invasive method based on inert gas throughflow theory. In vitro validation of the measurement system was performed using a lung gas exchange simulator. The accuracy and precision of the throughflow measurement system was assessed by comparing measured and target values for five simulated values of non-shunt cardiac output, from 2.88 to 9.86 l min(-1). This showed an overall mean bias of -0.031 min(-1) (range -0.00 to -0.101 min(-1)), with a mean coefficient of variation of the difference of 1.39% (1.20-1.93%). These results indicate that the measurement system is suitable for monitoring the non-shunt cardiac output in patients undergoing general anaesthesia using nitrous oxide throughflow.