TGN38, a transmembrane glycoprotein predominantly localized to the trans-Golgi network, is utilized to study both the structure and function of the trans-Golgi network (TGN). The effects of brefeldin A (BFA) on the TGN were studied in comparison to its documented effects on the Golgi cisternae. During the first 30 min of BFA treatment, the TGN loses its cisternal structure and extends as tubules throughout the cytoplasm. By 60 min, it condenses into a stable structure surrounding the microtubule-organizing center. By electron microscopy, this structure appears as a population of large vesicles, and by immunolabeling, most of these vesicles contain TGN38. TGN38 cycles to the plasma membrane and back, which is shown by addition of TGN38 luminal domain antibodies directly to cell culture media. This results in rapid uptake of antibodies which label the TGN within 30 min, both in its native and BFA-induced conformation. A number of transmembrane proteins have been shown to take this cycling pathway, but TGN38 is unique in that it is the only one predominantly localized to the TGN. To investigate the cycling of TGN38, the endocytic pathway was labeled by internalization of Lucifer Yellow, and in the presence of BFA there was partial colocalization with TGN38. Further studies were carried out in which microtubules were depolymerized, resulting in dispersal of Golgi elements and inhibition of transport from endosomes to lysosomes. TGN38 cycling continues in the absence of microtubules. Taken together, these studies indicate that TGN38 returns from the plasma membrane via the endocytic pathway. We conclude that the TGN is structurally and functionally distinct from the Golgi cisternae, indicating that different molecules control membrane traffic from the Golgi cisternae and from the TGN.