Brefeldin A is a fungal metabolite that has profound and dramatic effects on the secretory pathway in mammalian cells (for review and references, see Wood et al., 1991; Lippincott-Schwartz et al., 1991; Hunziker et al., 1991). It not only inhibits secretion but also causes massive morphological changes: typically, the Golgi apparatus disintegrates and many Golgi enzymes are redistributed to the endoplasmic reticulum (ER). Remarkably, these changes are fully reversed when the drug is removed. These findings have generated excitement, because they suggest that an analysis of brefeldin action may help to reveal the general principles that govern the formation and maintenance of the organelles of the secretory pathway, and the rules that dictate the orderly traffic of transport vesicles between them. Recent results indicate that brefeldin has multiple, species-specific effects on vesicular transport that suggest several distinct sites of action in the endomembrane system.