We are concerned with the mechanisms involved in the directed migration of eukaryotic cells. Previously we found that, inside cells at the edge of an experimental wound, the Golgi apparatus and the microtubule-organizing center were rapidly repositioned forward of the nucleus in the direction of subsequent cell migration into the wound. This repositioning was proposed to serve the purpose of introducing new membrane mass at the leading edge of the cell, by directing Golgi apparatus-derived vesicles bound for the plasma membrane to that edge. We now provide evidence to support this proposal. Cultured fibroblastic cells at the edge of a wound were infected with a temperature-sensitive mutant (0-45) of vesicular stomatitis virus. It is known that the G-protein, an integral membrane protein of the virus, is synthesized and remains in the rough endoplasmic reticulum at the nonpermissive temperature, but when the infected cells are shifted to the permissive temperature, the G-protein moves through the Golgi apparatus to the plasma membrane. By immunofluorescence microscopy, we here show that the first appearance of the G-protein at the cell surface corresponds to the leading edge of the motile cell. These observations are incorporated into a coherent scheme for the mechanisms involved in cell migration.