Xenopus laevis oocytes are an outstanding heterologous expression system for the investigation of ion channels. However, oocytes express an amazing variety of endogenous ion channels that can severely interfere with electrophysiological measurements. It is therefore necessary to be aware of the channels present in the oocyte and to be able to exclude artifacts they might cause during the analysis of heterologously expressed ion channels. Research on Xenopus endogenous ion channels has started over 30 years ago, and many channels have been described since then. This does not only include voltage-gated channels conducting Na(+), K(+), Ca(2+), and Cl(-), but also ion channels activated by ligand binding such as ionotropic neurotransmitter receptors. Furthermore, there are other channels such as those triggered by changes in osmolarity or mechanical stress, as well as conductances caused by yet uncharacterized molecules. Here, we present an overview of ion channels endogenous to the oocyte described in the literature so far, and provide procedures and methods to abolish or minimize their impact on electrophysiological recordings of exogenous channels.