THE fungal metabolite brefeldin A is a powerful tool for investigating membrane traffic in eukaryotic cells¹. The effects of brefeldin A on traffic are partly explained by its ability to prevent binding of cytosolic coat proteins onto membranes^2–5. The non-clathrin coatomer complex^6,7 binds reversibly to Golgi membranes in a GTP-controlled cycle^8–10. The low-molecular-mass GTP-binding protein ADP-ribosylation factor (ARF), which also associates reversibly with Golgi membranes^11,12, is required for coatomer binding¹³ and probably accounts for the control by guanine nucleotide of the coatomer–membrane interaction. Brefeldin A prevents the assembly of coatomer onto the membrane by inhibiting the GTP-dependent interaction of ARF with the Golgi membrane¹³, but the nature of this interaction has not been established. Here we demonstrate that Golgi membranes can specifically catalyse the exchange of GTP onto ARF and that brefeldin A prevents this function.