Endurance exercise training induces mitochondrial biogenesis in skeletal muscle. The peroxisome proliferator activated receptor co‐activator 1α (PGC‐1α) 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‐1α 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‐1α transcription (10‐ to > 40‐fold) and mRNA content (7‐ to 10‐fold), peaking within 2 h after exercise. Activation of PGC‐1α 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‐1α in cell culture. HKII, mitochondrial transcription factor A, peroxisome proliferator activated receptor α, and calcineurin Aα and Aβ 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‐1α transcription and mRNA content in human skeletal muscle. Consistent with its role as a transcriptional coactivator, these findings suggest that PGC‐1α may coordinate the activation of metabolic genes in human muscle in response to exercise.