ClC-2 is a chloride channel expressed in various epithelial and neuronal tissues. It slowly activates at voltages negative to the chloride reversal potential and is thus perfectly suited for extruding chloride ions from the cytoplasm. The carboxy-terminus of ClC channels contains cystathionine-ß-synthase (CBS) domains that have long been thought to be involved in regulation of ClC proteins by nucleotides. It has been noted for ClC-2 that gating can be modified by changes in the internal concentration of nucleotides such as Adenosine-tri- (ATP), -di- (ADP) or -monophosphate. However, a detailed analysis on the concentration dependence and a model explaining the effect of ATP on ClC-2 is missing so far. We investigated the effect of nucleotides using whole-cell and single channel patch clamp analyses using inducible stable cell lines expressing human ClC-2. Comparing internal ATP concentrations ([ATP]_i) from 0 up to the physiologically significant 5 mM revealed a slight shift of activation curves to more negative potentials. ClC-2 activates on a biexponential time course providing time constants of fast and slow activation. Whereas fast activation time constants were independent of [ATP]_i, slow time constants increased by up to ~2.5-fold with an apparent K_D of about 1 mM. Varying [ADP]_i demonstrated a similar behaviour on activation although with no effect on deactivation kinetics. Truncating the carboxy terminus (H573X) abolished nucleotide dependence altogether. Dwell time analysis of single channel currents led to the discovery of an ATP-dependent long lasting closed state. Our results indicate direct interaction of the carboxy-terminus and nucleotides such as ATP. It seems as if the slow gate can be "locked" into a nucleotide-bound closed or open state based on our modelling of whole cell currents. Seemingly, the effect of nucleotides on ClC-2 gating can be adequately described by a four state model for the slow gate.