Mouse oocytes spontaneously undergo meiotic maturation when removed from the ovary and cultured in vitro. This paper describes the changes in cell surface membrane potential from the germinal vesicle (GV) stage (- 46 mV, inside negative), through germinal vesicle breakdown (GVBD) (- 31 mV), to the polar body (PB) stage (- 17 mV). Radioisotope tracer flux experiments established the total exchangeable internal K at the GV stage in 190 mM, the K efflux rate coefficient as 6.7 X 10(-5) sec-1) and the K permeability as 46.7 X 10(-8) cm sec-1. In comparison with previous results with PB stage oocytes, this suggests that the membrane potential depolarization is caused largely by a decrease in K permeability. When GVBD is blocked with 0.11 mM dibutyryl cyclic AMP, the membrane potential does not depolarize. This suggests a linkage between depolarization and GVBD. However, incubation in 80 mM K medium, which depolarizes the oocyte, does not trigger GVBD. It is concluded that membrane depolarization by itself is not sufficient to cause GVBD.