Decreases in the intracellular concentrations of both K⁺ and Cl^- have been implicated in playing a major role in the progression of apoptosis, but little is known about the temporal relationship between decreases in electrolyte concentration and the key events in apoptosis and there is no information about how such decreases affect different intracellular compartments. Electron probe X-ray microanalysis was used to determine changes in element concentrations (Na, P, Cl and K) in nucleus, cytoplasm, and mitochondria in U937 cells undergoing UV-induced apoptosis. In all compartments the initial stages of apoptosis were characterised by decreases in [K] and [Cl]. The largest decreases in these elements were in the mitochondria and occurred before the release of cytochrome c. Initial decreases in [K] and [Cl] also preceded apoptotic changes in the nucleus. In the later stages of apoptosis the [K] continued to decrease whereas that of Cl began to increase towards control levels and was accompanied by an increase in [Na]. In the nucleus these increases coincided with poly(ADP-ribose) polymerase cleavage, chromatin condensation, and DNA laddering. The cytoplasm was the compartment least affected, the pattern of change of Cl was similar to those in other compartments but the decrease in [K] was not significant until after active caspase-3 was detected. Our results support the concept that normotonic cell shrinkage occurs early in apoptosis, and demonstrate that changes in the intracellular concentrations of K and Cl precede apoptotic changes in the cell compartments studied.