1. Whole-cell patch clamp recordings were made from Chinese hamster ovary (CHO) cells stably expressing homomeric mouse Kv1.1 (delayed rectifier K+; mKv1.1) channels. The effects of internal application of a number of different peptides, based on part of the amino terminal sequence of the human Kv3.4 channel subunit (hKv3.4), were examined in order to determine their influence on N-type inactivation. 2. For the native hKv3.4 peptide, the association rate constant (kon) increased with membrane depolarization, whilst the dissociation rate constant (koff) had little dependence on voltage. This resulted in the apparent dissociation constant (KD) of the hKv3.4 peptide tending to increase with depolarization. 3. In general, kon increased and apparent KD decreased with positive charge of the hKv3.4 peptide variants; in peptides lacking a hydrophobic amino terminal this correlation was not maintained. In contrast, the rate of dissociation of the variant peptides (koff) was independent of net charge. 4. The blocking activity of the hKv3.4 peptide was not dependent on a disulphide bridge between cysteine residues C6 and C24 and the presence of cysteine residues in the hKv3.4 peptide was not a prerequisite for rapid inactivation. All cysteine-substituted variants, especially at C6, showed a more rapid recovery from inactivation than the hKv3.4 peptide. Substitutions at C24, and not C6, reduced kon. 5. The present results concerning the action of the mammalian hKv3.4 channel inactivation particle on mKv1.1 channels complement earlier models for the invertebrate Shaker K+ channel. It is proposed that the hydrophobic amino terminal region of the hKv3.4 inactivation peptide blocks the channel pore, whilst the adjacent positively charged region interacts with negative charges on the channel protein.