Tendons that store energy during locomotion, such as the equine superficial digital flexor tendon (SDFT) and human Achilles tendon, suffer a high incidence of central core degeneration which is thought to precede tendon rupture. Although energy storage contributes to the efficiency of locomotion, tendons are not perfectly elastic and some energy is lost in the form of heat. Recent studies have shown that the central core of equine SDFT reaches temperatures as high as 45 degrees C during high-speed locomotion. In this study, we test the hypothesis that hyperthermia causes tendon cell death and results in tendon central core degeneration. Tendon fibroblasts cultured from the core of the equine SDFT were subjected to a temperature of 45 degrees C in an in vitro system for 0-180 min, and cell survival fraction was measured and compared with that for equine dermal fibroblasts and a commercial rat kidney fibroblast cell line (NRK 49F). Tendon fibroblasts were significantly more resistant to hyperthermia than NRK 49F cells after 30, 45 and 60 min of heating and significantly more resistant than dermal fibroblasts after 45 and 60 min of heating. After 10 min of heating at 45 degrees C, the tendon fibroblast cell survival fraction was 91 +/- 4%, whereas heating for 10 min at 48 degrees C resulted in a drop in the cell survival fraction to 22 +/- 4%. In conclusion, while temperatures experienced in the central core of the SDFT in vivo are unlikely to result in tendon cell death, repeated hyperthermic insults may compromise cell metabolism of matrix components, resulting in tendon central core degeneration.