Fully grown oocytes of Xenopus laevis undergo resumption of the meiotic cycle when treated with the steroid hormone progesterone. Previous studies have shown that meiotic maturation results in profound downregulation of specific endogenous membrane proteins in oocytes. To determine whether the maturation impacts the functional properties of exogenously expressed membrane proteins, we used cut-open recordings from Xenopus oocytes expressing several types of Na⁺ and K⁺ channels. Treatment of oocytes with progesterone resulted in a downregulation of heterologously expressed Na⁺ and K⁺ channels without a change in the kinetics of the currents. The time course of progesterone-induced ion channel inhibition was concentration dependent. Complete elimination of Na⁺ currents temporally coincided with development of germinal vesicle breakdown, while elimination of K⁺ currents was delayed by ~2 h. Coexpression of human ₁-subunit with rat skeletal muscle -subunit in Xenopus oocytes did not prevent progesterone-induced downregulation of Na⁺ channels. Addition of 8-bromo-cAMP to oocytes or injection of heparin before progesterone treatment prevented the loss of expressed currents. Pharmacological studies suggest that the inhibitory effects of progesterone on expressed Na⁺ and K⁺ channels occur downstream of the activation of cdc2 kinase. The loss of channels is correlated with a reduction in Na⁺ channel immunofluorescence, pointing to a disappearance of the ion channel-forming proteins from the surface membrane.