This study investigates the poor reversibility of salmon calcitonin (sCT) binding to rat and human calcitonin receptors. Efficacy of CT and analogue peptides in 125 I-sCT binding competition and cAMP assays was compared with the dissociation kinetics of 125 I-labelled peptides. Assessment was performed on cells stably expressing either rat or human calcitonin receptors. Dissociation kinetics of the antagonists, sCT(8-32) and AC512, revealed that binding was rapidly and completely reversible at the receptors, despite high affinity binding, suggesting that poor reversibility required the active conformation of the receptor. G protein coupling was not essential as the dissociation kinetics of 125 I-sCT binding to cell membranes did not significantly alter in the presence of GTPγS. Time course experiments established that the transition to irreversibility was slow, while the reversible component of binding appeared to involve a single population of either receptor states or binding sites. Pre-bound 125 I- human CT dissociated rapidly from the receptors, indicating that not all agonists bound irreversibly. To identify structural features of sCT that contribute to its poor reversibility, dissociation kinetics of sCT analogues with various structural modifications were examined. Increasing truncation of N-terminal residues of sCT analogues led to a corresponding increase in the rate of peptide dissociation. Salmon CT peptides which had been substituted at the N-terminus by 13-21 residues of human CT (hCT) were equipotent with sCT in binding competition and cAMP accumulation assays but exhibited a dissociation rate similar to hCT. In contrast, despite lower affinity and efficacy at the receptors, the chimetic analogue sCT(1-16)-hCT(17-32) displayed poorly reversible binding, similar to sCT. Analysis of the dissociation kinetics of sCT analogues with differing α-helix forming potential indicated that the ability to form α-helical secondary structure was an important factor in the rate of ligand dissociation. We hypothesise that poor reversibility results from a conformational change in the receptor and/or ligand and that this is dependent, at least in part, on interaction with residues constrained within the α-helix of the peptide.