A burst of action potentials in hippocampal neurons is followed by a slow afterhyperpolarization (AHP) that serves to limit firing in response to sustained excitation. Previous studies in our laboratory have shown that the pattern of action potential firing can influence the magnitude and stability of the slow AHP (SFN Abstract, 2002). Here we present the results of studies addressing a potential role for hyperpolarization-activated (H) channels in plasticity of the slow AHP. Whole-cell recordings were obtained from CA1 pyramidal neurons in hippocampal slices from young rats (14-24 days) and action potentials were evoked by brief current injections in 50 Hz trains or a theta-burst pattern (50 spikes per trial). The slow AHP evoked by 50 Hz trains (train-AHP) gradually increased over the course of an experiment, whereas the slow AHP evoked by theta stimulation (theta-AHP) gradually decreased. Application of the H-channel blocker ZD7288 (25 µM) blocked the enhancement of the train-AHP after repeated stimulat...