Despite earlier notions that intracellular pH (pHi) was invariant with time, recent studies have documented pHi changes of from 0.1 to 1.6 U during metabolic and developmental transitions in a variety of cells. Here we review the evidence for pHi-mediated regulation of gamete activation, cellular dormancy, the cell cycle, and stimulus-response coupling. Intracellular Ca2+ level changes also accompany many of these same transitions, and mounting evidence suggests that pHi and Ca2+ changes can be interdependent, both in their mechanisms and their effects. Although the significance of such interactions is still largely unclear, one example--the pronounced pH dependence of Ca2+ binding by calmodulin--suggests their potential importance in metabolic regulation. Similar evidence suggests that pHi changes also influence intracellular adenosine 3',5'-cyclic monophosphate levels, and vice versa. Finally we show that changes in adenylate energy charge can significantly alter pHi. In light of these interactions--and because pHi, unlike most other effectors, does not require specialized receptors--we suggest that pHi functions as a synergistic messenger, providing a metabolic context within and through which the actions of other effectors are integrated.