Cooling is accepted as a practical way of lowering cell metabolism in vein grafts during coronary by-pass surgery. We have previously shown that low temperature causes endothelial cells to become detached, both in in vitro and in vein graft. In this study we have looked at the effect of cold on the concentrations of intra- and extracellular electrolytes. Human endothelial cells were grown on titanium grids for electron microscopy. The cells were incubated for 30 min at 37 degrees, 20 degrees, and 4 degrees C with cell culture medium containing human serum, and at 20 degrees and 4 degrees C with heparinized sodium acetate solution with serum, frequently used for flushing and distending vein grafts. Freeze-dried cells were then subjected to elemental X-ray microanalysis. The ambient fluid was analysed by flame photometry. At 20 degrees and 4 degrees C, intracellular concentration of sodium increased, and potassium decreased, compared with controls (37 degrees C). The changes in sodium concentrations were aggravated when cell culture medium was replaced by heparinized sodium acetate. The intracellular chloride concentration did not change when cells were stored in cold cell culture medium. The extracellular concentration of potassium increased with increasing incubation time at 4 degrees C. The connection between these findings and cell detachment is discussed.