Intracellular proteins ligated to ubiquitin are degraded by the 26 S proteasome which is composed of the 20 S proteasome and a regulatory subunit complex. We have reported that ATP-dependent activity of the proteasome is activated periodically during the ascidian mitotic division cycle. In the present study, we examined changes in the activities and in the amounts of proteasomes during progression of the ascidian meiotic division cycle. During the metaphase-anaphase transition triggered by treatment with calcium ionophore, the activity of 26 S proteasome was found to be enhanced transiently and then decreased. The change in proteasome activity was completely abolished by pretreatment with a cell-permeable calcium chelating agent, BAPTA-AM, which indicates that proteasome activity is regulated by intracellular calcium mobilization. By immunoblot analyses, it was demonstrated that the 26 S proteasome underwent a change in amount in a manner similar to the change in its activities. The immunoblot analyses also indicated an inverse relation between the level of the 26 S proteasome and that of the 20 S proteasome throughout the cycle. These results, together with the fact that total amounts of the 26 S and 20 S proteasomes remain constant throughout the cycle, suggest that 26 S proteasome activity is regulated through interconversion between the 26 S and 20 S proteasomes induced by intracellular calcium mobilization during the meiotic metaphase-anaphase transition.