During behavior, hippocampal pyramidal cells emit high frequency bursts, modulated by the animal's location and the 7 Hz theta rhythm. During rest, CA1 EEG exhibits large irregular activity (LIA), containing sharp-wave/ripple complexes, during which pyramidal cells exhibit burst discharge. Aging results in altered intracellular calcium homeostasis, increased electrical coupling and reduced cholinergic modulation within CA1, all of which might affect burst discharge characteristics. During LIA, old rats exhibited more short (3-7 ms) inter-spike intervals, with no change in mean firing rate. During behavior induced theta rhythm, however, interval distributions were not affected by age. Thus, different mechanisms must underlie burst discharge in theta and LIA states. Moreover, age related changes in the cholinergic system appear not to play a major role in shaping the temporal discharge characteristics of CA1 pyramidal cells. The mechanism and significance of the higher frequency bursting in old rats during LIA remains to be determined.