Abstract We used intracellular recordings in a guinea pig brain slice preparation to characterize the actions of carbachol in neurons of the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited depolarization and rhythmic bursting activities in a majority of neurons (61.5%) through muscarinic receptors. Within the LSml, three types (I, II and III) of rhythmic bursting neurons could be distinguished on the basis of the properties (waveform, duration and amplitude) of their bursts induced by carbachol. In the three types of neurons, the frequency of the bursts depended on the membrane potential whereas the addition of tetrodotoxin to the bath did not suppress carbachol-induced rhythmic bursting activities. Therefore, these events were not driven by synaptic mechanisms but originated from endogenous properties of LSml neurons activated by carbachol. Thus, LSml neurons can be considered as conditional bursters.