The cellular mechanisms determining the firing patterns of gonadotropin-releasing hormone (GnRH) neurons are presently under intense investigation. In this study, we used GnRH-green fluorescent protein transgenic mice and perforated-patch electrophysiology to examine the role of small conductance calcium-activated potassium (SK) channels in determining the electrical excitability and burst firing characteristics of adult GnRH neurons. After establishing an appropriate protocol for examining the after-hyperpolarization potential (AHP) currents in GnRH neurons, the highly-selective SK channel blocker apamin was used to demonstrate that all GnRH neurons express functional SK channels (35.7±2.7 pA, mean decay time constant 2167 ms, apamin IC₅₀₌9.6 nM) and that this channel underlies 90% of the AHP in these cells. Current-clamp experiments showed that apamin-sensitive SK channels were tonically active in the majority (74%) of GnRH neurons, with apamin (100 nM) administration resulting in a mean 6.9±0.5 mV membrane depolarization. Apamin also elevated the firing rate of GnRH neurons, including increased burst frequency and duration in spontaneously bursting cells, as well as the ability of GnRH neurons to fire action potentials in response to current injection. In GnRH neurons activated by current injection, apamin significantly enhanced the amplitude of the afterdepolarization potential (ADP) following a single action potential and eliminated spike frequency adaptation. Together, these studies show that apamin-sensitive SK channels play a key role in restraining GnRH neuron excitability. Through direct modulation of the AHP, and indirect actions on the ADP, the SK channel exerts a powerful tonic influence upon the firing dynamics of GnRH neurons.