Dynorphin A (dynA) and related opioid peptides produce moderate analgesic effects with restricted types of pain stimuli that are often accompanied by a large variety of naloxone-insensitive biochemical and behavioural effects. In binding assays in vitro, dynA possesses a high affinity for mu-, delta- and kappa- opioid receptors with some selectivity for kappa sites, but it also binds to specific non-opioid sites. The involvement of the NMDA receptor has been suggested to explain some of the non-opioid effects of dynA and related peptides. In this article, Vijay Shukla and Simon Lemaire review the experimental evidence that suggests a role for the NMDA receptor in some of the pharmacological effects of dynA and related peptides.