Action potentials in neurones are followed by a hyperpolarization, which can last up to several seconds. This hyperpolarization has several phases that are mediated by the activation of different types of Ca(2+)-activated K+ currents. Patch-clamp studies have revealed two families of Ca(2+)-activated K+ channels of small (SKCa) and high (BKCa) conductance. Activation of BKCa channels contributes to action-potential repolarization, while SKCa channels are thought to underlie the afterhyperpolarization (AHP). In addition, AHPs in neurones can be divided into two distinct types that are easily separated by kinetic and pharmacological criteria. It is now clear that only one type of AHP can be explained by activation of SKCa channels while a new type of Ca(2+)-activated K+ channel underlies the other. Modulation of this channel by a range of transmitters is a key determinant of the excitability of many neurones.