Abstract: A method and apparatus for measuring metabolic rates such as oxygen consumption and carbon dioxide elimination rates of a patient intubated on a ventilator utilize an inspiration sample of gases provided by the ventilator and end-tidal and ambient pressure samples of expiration gases exhaled by the patient which are collected and analyzed to determine concentrations of the gases along with flow rate data to calculate the desired metabolic rates on a breath-by-breath flow weighted average basis. The ambient pressure sample is taken from a point prior to complete mixing of the gases exhaled in adjacent breaths and after the exhalation valve such that the pressure is essentially ambient room air pressure to facilitate accurate sychronization of the flow rate and concentration signals.