The electrical signalling properties of neurons originate largely from the gating properties of their ion channels, N-type inactivation of voltage-gated potassium (K-v) channels is the best-understood gating transition in ion channels, and occurs by a ’ball-and-chain’ type mechanism. In this mechanism an N-terminal domain (inactivation gate), which is tethered to the cytoplasmic side of the channel protein by a protease-cleavable chain, binds to its receptor at the inner vestibule of the channel, thereby physically blocking the pore(1,2). Even when synthesized as a peptide, ball domains restore inactivation in K-v channels whose inactivation domains have been deleted(2,3), Using high-resolution nuclear magnetic resonance (NMR) spectroscopy, we analysed the three-dimensional structure of the ball peptides from two rapidly inactivating mammalian K-v channels (Raw3 (K(v)3.4) and RCK4 (K(v)1.4)). The inactivation peptide of Raw3 (Raw3-IP) has a compact structure that exposes two phosphorylation sites and allows the formation of an intramolecular disulphide bridge between two spatially close cysteine residues. Raw3-IP exhibits a characteristic surface charge pattern with a positively charged, a hydrophobic, and a negatively charged region, The RCK4 inactivation peptide (RCK4-IP) shows a similar spatial distribution of charged and uncharged regions, but is more flexible and less ordered in its amino-terminal part.