In hippocampal pyramidal cells, a single action potential (AP) or a burst of APs is followed by a medium afterhyperpolarization (mAHP, lasting approximately 0.1 s). The currents underlying the mAHP are considered to regulate excitability and cause early spike frequency adaptation, thus dampening the response to sustained excitatory input relative to responses to abrupt excitation. The mAHP was originally suggested to be primarily caused by M-channels (at depolarized potentials) and h-channels (at more negative potentials), but not SK channels. In recent reports, however, the mAHP was suggested to be generated mainly by SK channels or only by h-channels. We have now re-examined the mechanisms underlying the mAHP and early spike frequency adaptation in CA1 pyramidal cells by using sharp electrode and whole-cell recording in rat hippocampal slices. The specific M-channel blocker XE991 (10 microm) suppressed the mAHP following 1-5 APs evoked by current injection at -60 mV. XE991 also enhanced the excitability of the cell, i.e. increased the number of APs evoked by a constant depolarizing current pulse, reduced their rate of adaptation, enhanced the after depolarization and promoted bursting. Conversely, the M-channel opener retigabine reduced excitability. The h-channel blocker ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride; 10 microm) fully suppressed the mAHP at -80 mV, but had little effect at -60 mV, whereas XE991 did not measurably affect the mAHP at -80 mV. Likewise, ZD7288 had little or no effect on excitability or adaptation during current pulses injected from -60 mV, but changed the initial discharge during depolarizing pulses injected from -80 mV. In contrast to previous reports, we found that blockade of Ca2+-activated K+ channels of the SK/KCa type by apamin (100-400 nm) failed to affect the mAHP or adaptation. A computational model of a CA1 pyramidal cell predicted that M- and h-channels will generate mAHPs in a voltage-dependent manner, as indicated by the experiments. We conclude that M- and h-channels generate the somatic mAHP in hippocampal pyramidal cells, with little or no net contribution from SK channels.