The present study was undertaken to examine the effect of 10 days of detraining levels of GLUT-4 protein expression and citrate synthase (CS) activity in the vastus lateralis of trained men. During the course of normal training, seven endurance-trained (T) men and eight age- and weight-matched active but untrained (UT) men underwent an oral glucose tolerance test (OGTT) after an overnight fast. Muscle samples were obtained from the vastus lateralis by needle biopsy for measurement of GLUT-4 protein and CS activity. The tests were repeated on six of the T subjects after 10 days of detraining (DT men). The area under the insulin response curve during OGTT was lower in T men than in DT and UT men (22.4 +/- 2.8, 32.1 +/- 5.9, and 39.9 +/- 4.7 x 10(-3) pmol.l-1.min-1, respectively; P < 0.05). There were no differences between groups in the glucose responses to OGTT. GLUT-4 protein levels and CS activity were higher in T men than in DT and UT men (GLUT-4: 4.37 +/- 0.40, 2.92 +/- 0.53, and 1.71 +/- 0.22 arbitrary standard units and CS: 47.12 +/- 4.75, 33.63 +/- 3.98, and 24.51 +/- 2.97 mumol.min-1.g-1, respectively; both P < 0.05). Muscle GLUT-4 protein content was correlated with CS activity in all three groups (r = 0.64, 0.68, and 0.96 for UT, T, and DT men, respectively). These results suggest that muscle GLUT-4 protein content and oxidative capacity undergo parallel adaptations after detraining in previously well-trained men.