Phasic activity in magnocellular neurosecretory cells is characterized by alternating periods of activity (bursts) and silence. During phasic bursts, action potentials are superimposed on plateau potentials that are generated by summation of depolarizing after-potentials. Dynorphin is copackaged in vasopressin neurosecretory vesicles that are exocytosed from magnocellular neurosecretory cell dendrites and terminals, and both peptides have been implicated in the generation of phasic activity. Here we show that somato-dendritic dynorphin release terminates phasic bursts by autocrine inhibition of plateau potentials in magnocellular neurosecretory cells recorded intracellularly from hypothalamic explants using sharp electrodes. Conditioning spike trains caused an activity-dependent reduction of depolarizing after-potential amplitude that was partially reversed by α-latrotoxin (which depletes neurosecretory vesicles) and by nor-binaltorphimine (κ-opioid receptor antagonist), but not by an oxytocin/vasopressin receptor antagonist or a μ-opioid receptor antagonist, indicating that activity-dependent inhibition of depolarizing after-potentials requires exocytosis of an endogenous κ-opioid peptide. κ-Opioid inhibition of depolarizing after-potentials was not mediated by actions on evoked after-hyperpolarizations since these were not affected by κ-opioid receptor agonists or antagonists. Evoked bursts were prolonged by antagonism of κ-opioid receptors with nor-binaltorphimine and by depletion of neurosecretory vesicles by α-latrotoxin, becoming everlasting in ∼50% of cells. Finally, spontaneously active neurones exposed to nor-binaltorphimine switched from phasic to continuous firing as plateau potentials became non-inactivating. Thus, dynorphin coreleased with vasopressin generates phasic activity through activity-dependent feedback inhibition of plateau potentials in magnocellular neurosecretory cells.