In many molluscs, it has been found that Ca² signaling pathway is involved in the resumption of meiotic maturation in oocytes. To better understand the possible role of Ca² signaling pathway in regulating meiotic maturation in oocytes of the northern quahog Mercenaria mercenaria, free extracellular Ca² , A23187 (calcium ionophore), verapamil (calcium channel blocker), and trifluoperazin (calmodulin antagonist) were used to incubate oocytes or serotonin-induced oocytes by pharmacological methods. Results show that extracellular Ca² (50 ∼ 10 µM) and A23187 (1 ∼ 10 µM) can stimulate the meiotic maturation. In addition, verapamil (1 ∼ 100 µM) and trifluoperazin (10 ∼ 1,000 µM) could inhibit serotonin-induced oocyte maturation. Therefore, Ca² is essential for the reinitiation of meiotic maturation in oocytes of the northern quahog. Moreover, an increase in [Ca² ]i can promote meiotic maturation.