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 Ca²⁺-activated K⁺ currents. Patch-clamp studies have revealed two families of Ca²⁺-activated K⁺ channels small (SK_Ca) and high (BK_Ca) conductance. Activation of BK_Ca channels contributes to action-potential repolarization, while SK_Ca 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 SK_Ca channels while a new type of Ca²⁺-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.