We recently reported that the entropy of the log interspike interval distribution recorded from hypothalamic neurones changes in response to physiological stimuli (Bhumbra & Dyball, 2004). It is thus important to know if subtle changes in interval distribution might have physiological significance. Dyball & McKenzie (2000) showed that the excitability of axons in magnocellular neurosecretory neurons of the supraoptic nucleus changes during trains of applied stimuli. There is an input to the arcuate nucleus (ARCN) from the suprachiasmatic nucleus (SCN). We thus examined whether short bursts of stimuli applied to the ARCN region that evoked antidromic action potentials in SCN cells might alter the excitability of their axons. Extracellular recordings were made from the suprachiasmatic nucleus (SCN) in 500 μm sagittal brain slices from Wistar rats and C57 black mice. All animals were humanely killed. Recordings were made from SCN cells that were antidromically activated from the arcuate nucleus (ARCN). Trains of 20 just-subthreshold stimuli at 50 Hz repeated at 1 s intervals decreased the threshold for antidromic activation. Whereas neither single subthreshold stimuli nor stimuli at the beginning of a train of 20 pulses evoked antidromic action potentials, later stimuli in the train did so. To test whether the increased excitability was mediated by local changes in potassium ion concentration at the stimulus site, we progressively increased the concentration of potassium in the perfusion solution while continuously stimulating at 1 Hz with a constant, subthreshold, intensity. Increased potassium concentration increased the number of antidromically activated spikes suggesting an enhanced excitability. By contrast, trains of 20 just-suprathreshold stimuli increased the threshold for antidromic activation in SCN cells. Whereas single suprathreshold stimuli invariably evoked an antidromic action potential, identical stimuli applied in trains of 20 pulses failed to evoke spikes after a few stimuli. After the first train of 20 pulses, the later stimuli in the trains fell below threshold and antidromic action potentials were not evoked. The decreased excitability of the SCN cells during train stimulation of the ARCN region may be due to a hyperpolarizing after-potential. The increased excitability is however probably mediated by local changes in potassium concentration. Both increased and decreased excitability may be important if short bursts of spikes can alter the likelihood of their propagation to axon terminals and influence release of neurotransmitters and thus transmission at synapses.