Abstract We have investigated the external Na + requirement for the activation of surf clam oocytes. Resumption of meiosis, evidenced by germinal vesicle breakdown (GVBD), may be triggered artificially by elevating the external K + concentration in the virtual absence of external Na + (< 5 μM). However, the kinetics of GVBD is slowed down considerably in the absence of Na +. The oocytes normally release acid upon activation through an Na +-dependent mechanism thought to cause an intracellular pH (pHi) rise. Addition of the weak base ammonia, which raises directly pHi, allows oocytes activated by K + in Na +-free medium to undergo GVBD with normal kinetics. We suggest that a rise of pHi is not essential for GVBD in surf clam oocytes but it normally contributes to accelerate the rate of reactions leading to activation.