1. The effects of Cs+ on the action potential, post-train after-hyperpolarization (AHP), Ca2+-dependent post-spike depolarizing after-potential (DAP) and phasic firing were examined during intracellular recordings from magnocellular neurosecretory cells (MNCs) in superfused rat hypothalamic explants. 2. Extracellular Cs+ reversibly inhibited (IC50, approximately 1 mM) DAPs, and associated after-discharges, that followed brief spike trains in each of sixteen cells tested. Although bath application of Cs+ also provoked a small reversible depolarization, inhibition of the DAP was retained when membrane voltage was kept constant by current injection. 3. Application of Cs+ had no significant effects on spike duration (n = 8), frequency-dependent spike broadening (n = 8), spike hyperpolarizing after-potentials (n = 14), or the amplitude of the isolated AHP (n = 7). Caesium-evoked inhibition of the DAP, therefore, does not result from diminished spike-evoked Ca2+ influx, and may reflect direct blockade of the conductance underlying the DAP. 4. Inhibition of the DAP was associated with an enhancement of the amplitude and duration of the AHP, indicating that the currents underlying the AHP and the DAP overlap in time following a train of action potentials, and that the relative magnitude of these currents is an important factor in determining the shape and time course of post-train after-potentials. 5. Bath application of Cs+ reversibly abolished phasic firing in each of seven cells tested. This effect was reversible and persisted at all subthreshold voltages tested. These results indicate that the current underlying the DAP is necessary for the genesis of plateau potentials and phasic firing in MNCs.