Transgenic mice, containing a chimeric gene in which the cDNA for phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C) (EC 4.1.1.32) was linked to the alpha-skeletal actin gene promoter, express PEPCK-C in skeletal muscle (1-3 units/g). Breeding two founder lines together produced mice with an activity of PEPCK-C of 9 units/g of muscle (PEPCK-C(mus) mice). These mice were seven times more active in their cages than controls. On a mouse treadmill, PEPCK-C(mus) mice ran up to 6 km at a speed of 20 m/min, whereas controls stopped at 0.2 km. PEPCK-C(mus) mice had an enhanced exercise capacity, with a VO(2max) of 156 +/- 8.0 ml/kg/min, a maximal respiratory exchange ratio of 0.91 +/- 0.03, and a blood lactate concentration of 3.7 +/- 1.0 mm after running for 32 min at a 25 degrees grade; the values for control animals were 112 +/- 21 ml/kg/min, 0.99 +/- 0.08, and 8.1 +/- 5.0 mm respectively. The PEPCK-C(mus) mice ate 60% more than controls but had half the body weight and 10% the body fat as determined by magnetic resonance imaging. In addition, the number of mitochondria and the content of triglyceride in the skeletal muscle of PEPCK-C(mus) mice were greatly increased as compared with controls. PEPCK-C(mus) mice had an extended life span relative to control animals; mice up to an age of 2.5 years ran twice as fast as 6-12-month-old control animals. We conclude that overexpression of PEPCK-C repatterns energy metabolism and leads to greater longevity.