Glucose and lactate profiles in Chinese hamster ovary cell cultures were accurately monitored in real time and in situ during three bioreactor batch cultures lasting 11, 15, and 15 days performed within a 60-day period. Monitoring was accomplished using in situ-collected mid-IR spectra analyzed with a priori one-time established partial least-squares regression models. The robustness of the technique was demonstrated by application of these models without modification after 2.3 yr. Neither recalibration nor instrument maintenance was required during the 2.3-yr period, except for the daily filling of liq. nitrogen for detector cooling during operation. The lactate calibration model yielded accurate abs. concn. estns. during each of the batch cultures with std. errors of est. from 1 to 3 mM. The a priori-established glucose calibration model yielded concn. estns. with an off-set, which was const. throughout a culture. Adjustment of the off-set before inoculation resulted in accurate concn. estns. with Std. errors of est. of approx. 1 mM for each of the bioreactor cultures. Sensitivity in detecting differences of 0.5 mM and selectivity against variation of one metabolite while the other was kept const. was demonstrated during std. addns. of either glucose or lactate. The sensor system proved to be reliable, simple, accurate, sterile, and capable of long-term automatic operation and is considered to be mature enough to be routinely applied for in situ (online) cell culture monitoring. [on SciFinder (R)]