1. The irreversible opioid receptor antagonist beta-chlornaltrexamine (beta-CNA) has been shown previously to have agonist activity in the guinea-pig ileum preparation. However, the receptor type or types mediating this effect have not been established. 2. In this study, the agonism of beta-CNA was investigated by use of the competitive antagonist 16-methylcyprenorphine (RX8008M). Non-cumulative concentration-effect curves for beta-CNA were displaced in a non-parallel fashion indicating that the agonism was mediated by both mu- and kappa-receptors. 3. In principle, expression of agonism by an irreversible receptor antagonist could compromise its use in estimating agonist dissociation constants (pKAs) due to desensitization operating in addition to receptor inactivation. For kappa-receptors, this possibility was checked by use of ethylketocyclazocine (EKC) to mimic the agonist effects of beta-CNA and test whether subsequent EKC concentration-effect curves were displaced. For mu-receptors it was necessary to perform more involved experiments in which [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGOL) was used as a standard agonist and its pKA was estimated under different conditions of beta-CNA incubation. 4. These analyses indicated that neither the mu- nor the kappa-receptor-mediated agonism of beta-CNA was associated with appreciable receptor desensitization. In turn it was concluded that the usefulness of beta-CNA as a pharmacological tool for the estimation of mu- and kappa-opioid receptor agonist dissociation constants is not compromised by the agonist effects that the compound demonstrates at these receptors.