The effects of training on silent mating-type information regulator 2 homolog 1 (SIRT1) activity and protein in relationship to peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and mitochondrial content were determined in human skeletal muscle. Six weeks of high-intensity interval training ( approximately 1 h of 10 x 4 min intervals at 90% peak oxygen consumption separated by 2 min rest, 3 days per week) increased maximal activities of mitochondrial enzymes in skeletal muscle by 28% to 36% (citrate synthase, beta-hydroxyacyl-coenzyme A dehydrogenase, and cytochrome c oxidase subunit IV) and PGC-1alpha protein (16%) when measured 4 days after training. Interestingly, total muscle SIRT1 activity (31%) and activity per SIRT1 protein (58%) increased despite decreased SIRT1 protein (20%). The present data demonstrate that exercise-induced mitochondrial biogenesis is accompanied by elevated SIRT1 activity in human skeletal muscle.