Starfish oocytes arrest at metaphase of the first meiotic division (MI arrest) in the ovary and resume meiosis after spawning into seawater. MI arrest is maintained by lower intracellular pH (pH(i)) and release from arrest by cellular alkalization. To elucidate pH(i) regulation in oocytes, we cloned the starfish (Asterina pectinifera) Na+/H+ exchanger 3 (ApNHE3) expressed in the plasma membrane of oocytes. The cytoplasmic domain of ApNHE3 contains p90 ribosomal S6 kinase (p90Rsk) phosphorylation sites, and injection of a constitutively active p90Rsk and the upstream regulator Mos to immature oocytes, stimulated an increase in pH(i). This increase was blocked by 5-(N-ethyl-N-isopropyl)-amiloride, a NHE inhibitor, and SL0101, a specific Rsk inhibitor. The MAPK kinase (MEK) inhibitor U0126 blocked the Mos-induced, but not the p90Rsk-induced, pH(i) increase, suggesting that the Mos-MEK-MAPK-p90Rsk pathway promotes ApNHE3 activation. In a cell-free extract, the Mos-MEK-MAPK-p90Rsk pathway phosphorylates ApNHE3 at Ser-590, -606, and -673. When p90Rsk-dependent ApNHE3 phosphorylation was blocked by a dominant-negative C-terminal fragment, or neutralizing antibody, the p90Rsk-induced pH(i) increase was suppressed in immature oocytes. However, ApNHE3 is up-regulated via the upstream phosphatidylinositol 3-kinase pathway before MAPK activation and the active state is maintained until spawning, suggesting that the p90Rsk-dependent ApNHE3 phosphorylation is unlikely to be the primary regulatory mechanism involved in MI arrest exit. After meiosis is completed, unfertilized eggs maintain their elevated pH(i) (similar to 7.4) until the onset of apoptosis. We suggest that the p90Rsk/ApNHE3-dependent elevation of pH(i) increases fertilization success by delaying apoptosis initiation.