Cytoplasmic pH (pHi) is a tightly regulated quantity, and changes in pHi tend to exert profound effects on the properties of cells. Substantial advances have been made in the last decade in understanding how pHi is regulated, principally through the development of techniques for measuring pHi accurately in living cells. Nonetheless, information about the roles played by changes in pHi under normal physiological circumstances, or about the effects of experimentally imposed changes in pHi on various cell properties, is still not very complete. In this paper, I review a number of experiments concerned with the effects of changes in pHi on the electrical properties of various excitable cells, such as neurons, muscle fibers, and oocytes. Because this review emphasizes intact cells and the possible physiological actions of pHi changes in modifying ion conductances rather than on the role of titratable chemical groups in the ion permeation process, I also discuss several aspects of pHi regulation in excitable cells as well as nonelectrophysiological effects of pHi changes. Understanding how excitable cells maintain and regulate cytoplasmic H+ ion activity is, for several reasons, essential for interpreting changes in pHi and their electrical effects.