Endogenous agonists acting at κ-opioid receptors modulate the discharge activity of hypothalamic supraoptic nucleus vasopressin cells in vivo. Phasic activity in vasopressin cells is known to depend critically on intrinsic mechanisms involving post-spike depolarizing after-potentials and we hypothesized that inhibition of phasic bursting by an endogenous κ-agonist may result from reducing the magnitude of depolarizing after-potentials. To investigate this possibility, intracellular sharp electrode recordings were obtained from supraoptic nucleus cells impaled in superfused explants of rat hypothalamus. Bath application of the selective κ-agonist, U50,488H (0.1–1 μM), decreased the spontaneous firing rate of magnocellular neurosecretory cells (by 94.0±4.5% at 1 μM, mean±SEM; P=0.02, n=4). U50,488H did not alter membrane potential (0.9±0.8 mV hyperpolarization at 1 μM, P=0.17, n=8) or input resistance (11.0±4.5% increase at 1 μM, P=0.09, n=5). U50,488H (0.1 and 1 μM, both n=5) reduced depolarizing after-potential amplitude (by 29.9±9.3 and 78.0±10.6%, respectively, P<0.001) in eight cells in which the baseline membrane potential was kept constant by dc-current injection and in which a depolarizing after-potential was evoked every 25–40 s by a brief (40–80 ms) train of 3–6 action potentials (the number of spikes in the trains was kept constant for each cell). Thus, κ-opioid receptor activation reduces depolarizing after-potential amplitude in supraoptic nucleus cells and this may underlie the reduction in burst duration of vasopressin cells caused by an endogenous κ-agonist in vivo.