In addition to action potential-evoked exocytotic release at neurohypophysial nerve terminals, the neurohormones arg-vasopressin (aVP) and oxytocin (OT) undergo Ca2+-dependent somatodendritic release within the supraoptic and paraventricular hypothalamic nuclei. However, the cellular and molecular mechanisms that underlie this release are not elucidated. In the present study, the whole cell patch-clamp technique was utilized in combination with high-time resolved measurements of membrane capacitance (Cm) and microfluorometric measurements of cytosolic free Ca2+ concentration ([Ca2+]in) to examine the Ca2+- and stimulus-dependence of exocytosis in the somata of magnocellular neurosecretory cells (MNCs) isolated from rat supraoptic nucleus (SON). Single depolarizing steps (20 ms) that evoked high voltage- activated (HVA) Ca2+ currents (ICa) and elevations in intracellular Ca2+ concentration were accompanied by an increase in Cm in a majority (40/47) of SON neurons. The Cm responses were composed of an initial Ca2+- independent, transient component and a subsequent, sustained phase of increased Cm (termed Cm) mediated by an influx of Ca2+, and increased with corresponding prolongation of depolarizing step durations (20 - 200 ms). From this relationship we estimated the rate of vesicular release to be 1533 vesicles/s. Delivery of neuron-derived action potential waveforms (APWs) as stimulus templates elicited ICa and also induced a Cm, provided APWs were applied in trains of greater than 13 Hz. A train of APWs modeled after the bursting pattern recorded from an OT- containing neuron during the milk ejection reflex was effective in supporting an exocytotic Cm in isolated MNCs, indicating that the somata of SON neurons respond to physiological patterns of neuronal activity with Ca2+-dependent exocytotic activity.