Phospholipase C-zeta (PLC zeta), a strong candidate of the egg-activating sperm factor, causes intracellular Ca2+ oscillations and egg activation, and is subsequently accumulated into the pronucleus (PN), when expressed in mouse eggs by injection of RNA encoding PLC zeta. Changes in the localization of expressed PLC zeta were investigated by tagging with a fluorescent protein. PLC zeta began to translocate into the PN formed at 5-6 h after RNA injection and increased there. Observation in the same embryo revealed that PLC zeta in the PN dispersed to the cytoplasm upon nuclear envelope breakdown and translocated again into the nucleus after cleavage. The dynamics was found in the second mitosis as well. When RNA was injected into fertilization-originated I-cell embryos or blastomere(s) of 2-8-cell embryos, the nuclear localization of expressed PLC zeta was recognized in every embryo up to blastocyst. Thus, PLC zeta exhibited alternative cytoplasm/nucleus localization during development. This supports the view that the sperm factor could control cell cycle-dependent generation of Ca2+ oscillations in early embryogenesis. (c) 2005 Elsevier Inc. All rights reserved.