This study tests the hypothesis that articular chondrocytes shift from a characteristically glycolytic to an oxidative energy metabolism during population expansion in monolayer. Bovine articular chondrocytes were cultured in monclayer under standard incubator conditions for up to 14 days. Cellular proliferation, oxygen consumption, lactate production, protein content, ROS generation and mitochondrial morphology were examined. Lactate release increased similar to 5-fold within 1 week, but this was limited to similar to 2-fold increase when normalized to cellular protein content. By contrast, per cell oxidative phosphorylation increased 98-fold in 1 week. The increase in oxidative phosphorylation was evident within 24 h, preceding cell proliferation and was associated with augmented reactive oxygen species generation. The autologous chondrocyte implantation procedure requires 14-21 days for population expansion. The alterations in metabolic phenotype we report within 7 days in vitro are thus pertinent to autologous chondrocyte implantation with significant implications for the chondrocyte functionality. (C) 2008 Elsevier Inc. All rights reserved.