Abstract: An on-line respirometer for measuring oxygen uptake by wastewater samples, plants, or animals due to respiration. A closed reaction vessel is adapted to receive and expel wastewater samples or the like. The vessel contains oxygen-sensing probes that constantly evaluate the concentration of oxygen either in the headspace of the vessel or in the samples contained in the slurry within the vessel. The oxygen that is consumed during the respiration of the test samples is immediately replaced by concurrent oxygen injection to maintain a constant oxygen concentration in the reaction vessel. This rapid oxygen replacement is accomplished through an oxygen feedback supply circuit that restores oxygen concentration to its original level in a matter of seconds. The respirometer uses a constant air flow system that allows for the precise oxygen level control. Direct measurement of the oxygen injected for equilibrium control is equated to the uptake.