Using the voltage clamp technique, the calcium-dependent transient outward chloride current in Xenopus oocytes was monitored during in vitro meiotic maturation induced by progesterone. This Cl conductance increased during the first 5-8 hours following progesterone stimulation. It then decreased until germinal vesicle breakdown (GVBD). Before GVBD, the Cl conductance could be activated by A23187 ionophore. After GVBD, the transient current totally disappeared, and only a single voltage-gated current could be observed.