High-density perfusion cultivation of mammalian cells can result in elevated bioreactor CO2 partial pressure (pCO(2)), a condition that can negatively influence growth, metabolism, productivity, and protein glycosylation. For BHK cells in a perfusion culture at 20 x 10(6) cells/mL, the bioreactor pCO(2) exceeded 225 mm Hg with approximate contributions of 25% from cellular respiration, 35% from medium NaHCO3, and 40% from NaHCO3 added for pH control. Recognizing the limitations to the practicality of gas sparging for CO2 removal in perfusion systems, a strategy based on CO2 reduction at the source was investigated. The NaHCO3 in the medium was replaced with a MOPS-Histidine buffer, while Na2CO3 replaced NaHCO3 for pH control. These changes resulted in 63-70% pCO(2) reductions in multiple 15 L perfusion bioreactors, and were reproducible at the manufacturing-scale. Bioreactor pCO(2) values after these modifications were in the 68-85 mm Hg range, pCO(2) reductions consistent with those theoretically expected. Low bioreactor pCO(2) was accompanied by both 68%-123% increased growth rates and 58%-92% increased specific productivity. Bioreactor pCO(2) reduction and the resulting positive implications for cell growth and productivity were brought about by process changes that were readily implemented and robust. This philosophy of pCO(2) reduction at the source through medium and base modification should be readily applicable to large-scale fed-batch cultivation of mammalian cells.