When extracts of mouse testis were Western-blotted against a monoclonal antibody which reacts with calmodulin in the presence of Ca2+, all calmodulin was associated with the macromolecules of molecular weight above 50 kDa. Immuno-electron microscopy of testes using this antibody indicated that calmodulin is localized at higher density in the nucleus and cytoplasm of germ cells during the developmental phase between pachytene and round spermatid, showing the highest level just before meiotic divisions. There was no special association of calmodulin to any organelles in these cells. Extremely low levels of calmodulin occurred in spermatogonia and other testicular tissue cells. Calmodulin decreased dramatically as spermatids underwent metamorphosis, becoming detectable only at the perinuclear space of sperm heads. Further relocation to the postacrosomal region occurred during sperm transit to the cauda epididymis. Immunodetection after the calmodulin overlay on ultrathin sections revealed a sharp increase of calmodulin immunogold deposits in the nuclei of spermatids accompanying their condensation. The results indicate that some calmodulin-binding proteins, but not calmodulin itself, accumulate in the nuclei during the final steps of spermiogenesis.