Oxygen is a key substrate in animal cell metab. and its consumption is thus a parameter of great interest for bioprocess monitoring and control. A system for measuring it based on an oxygen balance on the liq. phase was developed. The use of a gas-permeable membrane offered the possibility to provide the required quantity of oxygen into the culture, while avoiding problems of foaming or shear stress generally linked to sparging. This aeration system allowed moreover to keep a known and const. kLa value through cultures up to 400 h. Oxygen uptake rate (OUR) was measured online with a very good accuracy of +-5%, and the specific OUR for a CHO cell line was detd. during batch (growth phase) and continuous culture as, resp., equal to 2.85*10-13 and 2.54*10-13 mol O2 cell-1 h-1. It was also shown that OUR continuous monitoring gives actually more information about the metabolic state of the culture than the cell concn. itself, esp. during transition phases like the end of the growth phase in a batch culture. [on SciFinder (R)]