Conotoxin ImI is a specific marker of α7 nicotinic acetylcholine receptors. To study the role of aromatic indole group of tryptophan 10 in biological activity of ImI, the analogue containing tyrosine at this position was synthesized by solid-phase peptide synthesis. The analogue obtained, as well as its iodinated derivatives, were shown to be active against rat brain α7 acetylcholine receptor expressed in Xenopus oocytes. Attachment of bulky aromatic p-benzoylbenzoyl group to N-terminal α-amino group of iodinated [Tyr10]ImI only slightly affected the biological activity of the analogue. The data obtained suggest that indole ring of tryptophan 10 is not absolutely necessary for biological activity of conotoxin ImI, and that the N-terminus can accommodate a large aromatic group without loss of biological activity. Copyright (C) 1999 Elsevier Science Ltd.