The membrane input resistance (R(m)), potential (E(m)), and capacitance (C(m)) of fully grown Xenopus oocytes removed from their follicles have been measured. Before hormonal maturation, R(m), E(m), and C(m) were found to be: 1.86 +/- 0.63 Momega, -49 +/- 17 mV, and 11.9 +/- 4.8 microF/cm2 of apparent surface area. Long-term recording from oocytes in the presence of 1 microM progesterone reveal that during early GVBD (germinal vesicle breakdown) the membrane potential depolarized to about -10 mV, the apparent specific capacitance decreased to 1.5 to 3 microF/cm2, and the membrane resistance transiently increased by a factor of about 3. These results indicate that a decrease in surface area, a transient increase in membrane resistance, and a change in the ionic mechanisms for the membrane potential occur during progesterone-induced meiotic maturation. The continuous monitoring of these parameters has further shown that the major changes occur at about 50 to 60% of the time from progesterone application to GVBD, and that at the sensitivities used, generally 1.25 mV/mm, no changes are seen immediately upon application of the hormone.