Elevation of intracellular free calcium causes egg activation by initiating a cascade of interacting signaling pathways that, in unison, act to remodel the cytoplasmic compartment and the nuclear compartment of the egg. We show here that calcium/calmodulin-dependent protein kinase II (CaM kinase II) is tightly associated with the meiotic spindle and that 5 min after egg activation there is a transient, tight association of calmodulin (colocalized with CaM kinase II) on the meiotic spindle. These correlative observations caused us to test whether activation of CaM kinase II mediated the chromosomal transit into an anaphase configuration. We demonstrate that calcium and calmodulin, at physiological levels, along with ATP were capable of driving the spindle (with its associated CaM kinase II) into an anaphase configuration in a permeabilized egg system. The transit into anaphase was dependent on the presence of both calcium and calmodulin and occurred normally when they were present at a ratio of 4 to 1. Peptide and pharmacologic inhibitors of CaM kinase II blocked the transit into anaphase, both in the permeabilized egg system and in living eggs (inhibitors of protein kinase C did not block the transit into anaphase). Using a biochemical approach we confirm that CaM kinase II increases in activity 5 min after egg activation and that a second increase occurs 45 min after activation at the approximate time that the contractile ring of the second polar body is constricting. This corresponds to the approximate time when calmodulin and CaM kinase II colocalize at several points in the activated egg including the region containing midzone microtubules. CaM kinase II appears localized on midzone microtubules as soon as they form and may have a role in specifying the position of the contractile ring of the second polar body.