Abstract We examined the effect of interleukin-4 (IL-4) on matrix synthesis in chondrocytes under excessive mechanical stress in vitro. Chondrocytes from 7-day-old rat articular cartilage were incubated in the presence of rat IL-4 (0, 1, and 10 ng/ml) under a 5% CO2 atmosphere for 36 h. Cyclic mechanical stress (0.5 Hz, 7% elongation) was loaded using a Flexercell strain unit for 12, 16, and 24 h. Levels of messenger RNA (mRNA) for aggrecan (AGG), type II collagen (CII), fibronectin (FN), and integrin-β1 (INTb1) were measured by real-time reverse transcriptase polymerase chain reaction (RT-PCR) using GAPDH as the internal control. Mechanical stress for 16 h significantly decreased levels of mRNA for both AGG and CII (P < 0.01), but with rat IL-4 at doses of 1 or 10 ng/ml these levels recovered (P < 0.05). In addition, mRNA levels of FN and INTb1 were increased by IL-4 in chondrocytes under mechanical stress (P < 0.05). IL-4 protects chondrocytes loaded with excessive mechanical stress against degradation.