The relationships among fiber type, glucose transporter (GLUT-4) protein content, and glucose transport activity stimulated maximally with insulin and/or contractile activity were studied by use of the rat epitrochlearis (15% type I-20% type II2a-65% type IIb), soleus (84-16-0%), extensor digitorum longus (EDL, 3-57-40%), and flexor digitorum brevis (FDB, 7-92-1%) muscles. Insulin-stimulated 2-deoxy-D-glucose (2-DG) uptake was greatest in the soleus, followed (in order) by the FDB, EDL, and epitrochlearis. On the other hand, contractile activity induced the greatest increase in 2-DG uptake in the FDB, followed by the EDL, soleus, and epitrochlearis. The effects of insulin and contractile activity on 2-DG uptake were additive in all the muscle preparations, with the relative rates being FDB greater than soleus greater than EDL greater than epitrochlearis. Quantitation of the GLUT-4 protein content with the antiserum R820 showed the following pattern: FDB greater than soleus greater than EDL greater than epitrochlearis. Linear regression analysis showed that whereas a relatively low and nonsignificant correlation existed between GLUT-4 protein content and 2-DG uptake stimulated by insulin alone, significant correlations existed between GLUT-4 protein content and 2-DG uptake stimulated either by contractions alone (r = 0.950) or by insulin and contractions in combination (r = 0.992). These results suggest that the differences in maximally stimulated glucose transport activity among the three fiber types may be related to differences in their content of GLUT-4 protein.