Patch-clamp techniques were used in primary cultures of rat lactotropes and the rat pituitary clonal cell line GH3 to determine the population characteristics of spontaneous action potentials and the nature of the currents responsible for repolarization of spontaneous action potentials. Spontaneous action potentials were observed in 75% of lactotropes (74/99) and 80% of GH3 cells (42/51). Lactotropes exhibited broad and shallow action potentials (average duration 460 ms, peak –17 mV, slope of upstroke 0.5 mV/ms) compared to the GH3 cells which displayed narrow and tall action potentials (average duration 177 ms, peak –10 mV, slope of upstroke 1.6 mV/ms). Blockers of potassium currents were used to determine the role of specific potassium currents in the repolarization process. Spontaneous action potentials in lactotropes were largely unaffected by 4-aminopyridine (4AP), charybdotoxin, and apamin. Tetraethylammonium (TEA) caused only an small increase in peak amplitude and, in a subset of cells, a small increase in duration. In contrast, in GH3 cells, TEA, 4AP, charybdotoxin, and apamin all caused a significant increase in duration, while TEA and charybdotoxin also caused an increase in peak amplitude. Further, apamin caused a positive shift in the afterhyperpolarization voltage. In lactotropes, strong buffering of intracellular calcium with calcium chelators (EGTA or BAPTA) caused a profound increase in action potential duration. Thus, repolarization of action potentials in lactotropes is a calcium-dependent process, but unlike GH3 cells, is not mediated by calcium-dependent potassium currents, nor is it strongly influenced by voltage-dependent potassium currents.