To determine the role of intracellular [Ca.sup.2+] in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular [Ca.sup.2+], blocking of [Ca.sup.2+] channels, and inhibition of microfilaments, calmodulin, and intracellular [Ca.sup.2+] release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free [Ca.sup.2+] concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell-cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free [Ca.sup.2+] provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular [Ca.sup.2+]. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free [Ca.sup.2+] and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular [Ca.sup.2+] specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.