Understanding the mechanisms that influence the initiation of action potentials in single neurons is an important step in determining the way information is processed by neural networks. Therefore, we have investigated the properties of action potential thresholds for hippocampal neurons using in vivo intracellular recording methods in Sprague-Dawley rats. The use of in vivo recording has the advantage of the presence of naturally occurring spatio-temporal patterns of synaptic activity which lead to action potential initiation. We have found there is a large variability in the threshold voltage (5.7+/-1.7 mV; n=22) of individual action potentials. We have identified two separate factors that contribute to this variation in threshold: (1) fast rates of membrane potential change prior to the action potential are associated with more hyperpolarized thresholds (increased excitability) and (2) the occurrence of other action potentials in the 1 s prior to any given action potential is associated with more depolarized thresholds (decreased excitability). We suggest that prior action potentials cause sodium channel inactivation that recovers with approximately a 1-s time constant and thus depresses action potential threshold during this period.