Glucagon-like peptide-1 (7-36 amide) (GLP-1) is known to increase insulin release when given as a bolus in the fasted and fed state. GLP-1 also increases glucose uptake and lipid synthesis in cultured adipocytes. In this study we investigated the effects of GLP-1 on glucose uptake and on the levels of expression of the facilitative glucose transporters, GLUT1 and GLUT4, in fully differentiated 3T3-L1 adipocytes. Cells were incubated with GLP-1 (10 nM) with or without insulin (10 and 100 nM) for 24 h. Under these conditions, GLP-1 alone caused an increase in basal and acute insulin-stimulated glucose uptake along with an increase in GLUT1 and GLUT4 protein levels. However, there was no change in the expression of GLUT1 and GLUT4 mRNAs. In the absence of GLP-1, prolonged exposure to insulin caused a marked reduction in the levels of GLUT4 mRNA and protein, and an inhibition of glucose uptake after an acute exposure to insulin. This insulin-induced down-regulation of GLUT4 was prevented when GLP-1 was present during the 24-h treatment. In contrast, the acute insulin-stimulated glucose uptake could not be restored by GLP-1. GLP-1 is therefore the first gut hormone shown to be capable of modulating glucose transporter levels in cultured adipocytes.