During the spawning process in starfish, oocytes are arrested at metaphase of meiosis I (MI) within the ovary, and reinitiate meiosis only after they have been released into the seawater. However, this arrest does not occur if the ovary is removed from the animal. As the pH of the coelomic fluid is buffered by CO₂/H /HCO₃^-, we investigated the involvement of gas concentrations in MI arrest. In vivo, the CO₂ level in the coelomic fluid was high (∼1.5% vs. 0.04% in air) and the O₂ level was low (0.1–1.0% vs. ∼20% in air). When these gas conditions were reproduced in isolated coelomic fluid or seawater, ovarian oocytes arrested at MI, just as in vivo. Isolated oocytes from the ovary required the similar high CO₂ and low O₂ level to remain arrested in MI and had an intracellular pH of ∼6.9. Intracellular pH increased to ∼7.3 when oocytes were transferred to seawater equilibrated with air, a condition that mimics that of spawning. We used ammonium acetate to clamp intracellular pH at different levels and found that MI arrest occurred when intracellular pH was ∼6.9. Our results support the idea that high CO₂ and low O₂ in the ovarian environment lead to low intracellular pH and MI arrest, while spawning into the seawater with low CO₂ and high O₂ results in high intracellular pH and release from MI arrest. The biological significance of MI arrest is that oocytes are spawned into seawater at the optimal physiological state of MI when the least polyspermy occurs.