The insulin-dependent uptake of glucose by adipose and muscle tissues is accomplished through the regulated vesicle trafficking of the GLUT4 glucose transporter to the plasma membrane. The Sec1p homologue Munc18c is believed to play a central role in the docking of GLUT4 vesicles by controlling SNARE complex assembly. In the present study we have examined the function of SM proteins in insulin-stimulated GLUT4 trafficking in adipocytes. Syntaxin4 at the plasma membrane is not dependent on the presence of Munc18c. We found that absence of Munc18c did not affect GLUT4 externalization at the plasma membrane and GLUT4 trafficking was normal in the absence of Munc18c and functional Syntaxin2, known to be associated with Munc18b. Syntaxin4 demonstrates a robust interaction with Munc18c but not either Munc18a or Munc18b in 3T3-L1 adipocytes. However, Munc18a and Munc18b exhibited weak interaction with Syntaxin4 in the background of absence of Munc18c. These data suggest that Syntaxin4 may play an important role in insulin-stimulated GLUT4 trafficking and its interaction with SM proteins are complementary.