Big-K(+) conductance (BK)-channel mediated fast afterhyperpolarizations (AHPs) following action potentials are reduced after eyeblink conditioning. Blocking BK channels with paxilline increases evoked firing frequency in vitro and spontaneous pyramidal activity in vivo. To examine how increased excitability after BK-channel blockade affects learning, rats received bilateral infusions of paxilline, saline, or nothing into hippocampal CA1 prior to trace eyeblink conditioning. The drug group was slower to acquire the task, but learning was not completely impaired. This suggests that nonspecific increases in excitability and baseline neuronal firing rates caused by in vivo blockade of the BK channel may disrupt correct processing of inputs, thereby impairing hippocampus-dependent learning.