Recent studies using the fungal metabolite brefeldin A (BFA) have provided important insights into the dynamics and the organization of the ER/Golgi membrane system. Here we examined the effect of BFA on the functional integrity of the distal part of the secretory pathway, i.e., transport between trans-Golgi cisternae and the cell surface. To assay export via the constitutive pathway, we followed the movement of vesicular stomatitis virus (VSV) G glycoprotein that had been accumulated in the trans-Golgi network (TGN) by incubation of infected BHK-21 cells at 20 degrees C. Addition of BFA rapidly and reversibly inhibited cell surface transport of G protein. The block to secretion was not due to redistribution of externalized G protein to internal pools. It was also not due to collapse of TGN to the ER, since VSV G protein blocked in treated cells resided in compartments that were distinct from the ER/Golgi system. Similar effects were found with a bulk-flow marker: BFA blocked constitutive secretion of glycosaminoglycan chains that had been synthesized and sulfated in the trans-Golgi cisternae. To examine export via the regulated secretory pathway, we assayed secretion of [35S]SO4 labeled secretogranin II from PC12 cells, a marker that has been used to study secretory granule budding from the TGN (Tooze, S. A., U. Weiss, and W. B. Huttner. 1990. Nature [Lond.]. 347:207-208). BFA potently inhibited secretion of sulfated secretogranin II induced by K+ depolarization. Inhibition was at the level of granule formation, since BFA had no effect on regulated secretion from preformed granules. Taken together, the results suggest that BFA blocks export via both the constitutive and the regulated pathways. In contrast, endocytosis and recycling of VSV G protein were not blocked by BFA, consistent with previous studies that endocytosis is unaffected (Misumi, Y., Y. Misumi, K. Miki, A Takatsuki, G. Tamura, and Y. Ikehara. 1986. J. Biol. Chem. 261:11398-11403). These and earlier results suggest that the exo/endocytic pathway of mammalian cells consist of two similar but distinct endomembrane systems: an ER/Golgi system and a post-Golgi system. BFA prevents forward transport without affecting return traffic in both systems.