Endurance exercise training induces mitochondrial biogenesis in skeletal muscle. The peroxisome proliferator activated receptor co-activator 1a (PGC-1a) has recently been identified as a nuclear factor critical for coordinating the activation of genes required for mitochondrial biogenesis in cell culture and rodent skeletal muscle. To determine whether PGC-1a transcription is regulated by acute exercise and exercise training in human skeletal muscle, seven male subjects performed 4 weeks of one-legged knee extensor exercise training. At the end of training, subjects completed 3 h of two-legged knee extensor exercise. Biopsies were obtained from the vastus lateralis muscle of both the untrained and trained legs before exercise and after 0, 2, 6 and 24 h of recovery. Time to exhaustion (2 min maximum resistance), as well as hexokinase II (HKII), citrate synthase and 3-hydroxyacyl-CoA dehydrogenase mRNA, were higher in the trained than the untrained leg prior to exercise. Exercise induced a marked transient increase (P < 0.05) in PGC-1a transcription (10- to > 40-fold) and mRNA content (7- to 10-fold), peaking within 2 h after exercise. Activation of PGC-1a was greater in the trained leg despite the lower relative workload. Interestingly, exercise did not affect nuclear respiratory factor 1 (NRF-1) mRNA, a gene induced by PGC-1a in cell culture. HKII, mitochondrial transcription factor A, peroxisome proliferator activated receptor a, and calcineurin Aa and Aß mRNA were elevated (˜2- to 6-fold; P < 0.05) at 6 h of recovery in the untrained leg but did not change in the trained leg. The present data demonstrate that exercise induces a dramatic transient increase in PGC-1a transcription and mRNA content in human skeletal muscle. Consistent with its role as a transcriptional coactivator, these findings suggest that PGC-1a may coordinate the activation of metabolic genes in human muscle in response to exercise.