A regulatory role for intracellular free calcium has been suggested in cell division and intercellular communication via gap junctions. Previous measurements of cytoplasmic free Ca2+ during cell division and uncoupling have been made with the Ca2+-sensitive photoprotein aequorin. We now report experiments using Ca2+-sensitive microelectrodes to monitor free Ca2+ in the cells and intercellular fluid of early embryos of the amphibian Xenopus laevis. In addition to measuring basal levels, we have looked for changes in free Ca2+ during cell division and during electrical uncoupling of the normally coupled embryonic cells, induced by acidification of the intracellular medium.