In fat and muscle cells, insulin stimulates the movement to and fusion of intracellular vesicles containing GLUT4 with the plasma membrane, a process referred to as GLUT4 translocation. Previous studies have indicated that Akt [also known as PKB (protein kinase B)] phosphorylation of AS160, a GAP (GTPase-activating protein) for Rabs, is required for GLUT4 translocation. The results suggest that this phosphorylation suppresses the GAP activity and leads to the elevation of the GTP form of one or more Rabs required for GLUT4 translocation. Based on their presence in GLUT4 vesicles and activity as AS160 GAP substrates, Rabs 8A, 8B, 10 and 14 are candidate Rabs. Here, we provide further evidence that Rab10 participates in GLUT4 translocation in 3T3-L1 adipocytes. Among Rabs 8A, 8B, 10 and 14, only the knockdown of Rab10 inhibited GLUT4 translocation. In addition, we describe the subcellular distribution of Rab10 and estimate the fraction of Rab10 in the active GTP form in vivo. Approx. 5% of the total Rab10 was present in GLUT4 vesicles isolated from the low-density microsomes. In both the basal and the insulin state, 90% of the total Rab10 was in the inactive GDP state. Thus, if insulin increases the GTP form of Rab10, the increase is limited to a small portion of the total Rab10. Finally, we report that the Rab10 mutant considered to be constitutively active (Rab10 Q68L) is a substrate for the AS160 GAP domain and, hence, cannot be used to deduce rigorously the function of Rab10 in its GTP form.