. Citrate synthase expression and enzyme activity after endurance training in cardiac and skeletal muscles. J Appl Physiol 94: 555-560, 2003; 10.1152/ japplphysiol.00821.2002.-The present study was designed to examine the acute and chronic effects of endurance treadmill training on citrate synthase (CS) gene expression and enzymatic activity in rat skeletal and cardiac muscles. Adult rats were endurance trained for 8 wk on a treadmill. They were killed 1 h (T1, n ϭ 8) or 48 h (T48, n ϭ 8) after their last bout of exercise training. Eight rats were sedentary controls (C) during the training period. CS mRNA levels and enzymatic activities of the soleus and ventricle muscles were determined. Training resulted in higher CS mRNA levels in both the soleus muscles (21% increase in T1; 18% increase in T48, P Ͻ 0.05) and ventricle muscles (23% increase in T1; 17% increase in T48, P Ͻ 0.05) when compared with the C group. The CS enzyme activities were 42 (P Ͻ 0.01) and 25% (P Ͻ 0.01) greater in the soleus muscles of T1 and T48 groups, respectively, when compared with that of the C group. Soleus CS enzyme activity was significantly greater in the T1 vs. T48 groups (P Ͻ 0.05). However, no appreciable alterations in CS enzyme activities were observed in the ventricle muscles in both training groups. These findings suggest differential responses of skeletal and cardiac muscles in CS enzymatic activity but similar responses in CS gene expression at 1 and 48 h after the last session of endurance training. Moreover, our data support the existence of an acute effect of exercise on the training-induced elevation in CS activity in rat soleus but not ventricle muscles. physical activity; oxidative enzyme; gene transcriptional expression; soleus muscle; heart muscle CITRATE SYNTHASE (CS) is one of the key regulatory enzymes in the energy-generating metabolic pathway that catalyzes the condensation of oxaloacetate and acetyl coenzyme A to form citrate in the tricarboxylic acid cycle. It has been extensively used as a metabolic marker in assessing oxidative and respiratory capacity (22). More than 30 years ago, Holloszy and his colleagues (8) reported that endurance exercise training increases the oxidative enzyme activities in skeletal muscle. After that, numerous studies were conducted to examine the effect of exercise training on muscle oxidative capacity and metabolic characteristics (7,15,20,22,29). It is generally recognized that the skeletal muscle oxidative capacity and the activities of mitochondrial enzymes are elevated by endurance exercise training.