AIM: The mitochondrial uncoupling protein-3 (UCP3) is able to lower the proton gradient across the inner mitochondrial membrane, thereby uncoupling substrate oxidation from ATP production and dissipating energy as heat. What the effect of endurance training on UCP3 is, is still controversial. Endurance-trained athletes are characterized by lower levels of UCP3, but longitudinal studies in rodents reported no effect of endurance training on muscular UCP3 levels. Here, we examined the effect of a 2-week training programme on skeletal muscle UCP3 protein content in untrained human subjects, and hypothesized that UCP3 will be reduced after the training programme. METHODS: Nine untrained men [age: 23.3 +/- 3.2 years; BMI: 22.6 +/- 2.6 kg m(-2); maximal power output (W(max)): 3.8 +/- 0.6 W kg(-1) body weight] trained for 2 weeks. Before and at least 72 h after the training period, muscle biopsies were taken for determination of UCP3 protein content. RESULTS: UCP3 protein content tended to be lower after the training programme [95 +/- 10 vs. 109 +/- 12 arbitrary units (AU), P = 0.08]. Cytochrome c content tended to increase with 33% in response to endurance training (52 +/- 6 vs. 39 +/- 6 AU, P = 0.08). The ratio UCP3 relative to cytochrome c tended to decrease significantly upon endurance training (2.0 +/- 0.4 vs. 3.2 +/- 0.6 AU, P = 0.01). CONCLUSION: A short-term (2-week) endurance training programme decreased UCP3 protein levels and significantly reduced the ratio of UCP3 to cytochrome c.