In vitro neurophysiological studies have demonstrated a circadian rhythm with a period of 24 h in spontaneous neuronal discharge frequency within the rodent suprachiasmatic nucleus (SCN) circadian clock. This report examines the 'circadian' firing rate rhythm in the SCN of the homozygous Tau-mutant Syrian hamster which expresses a short period behavioural rhythm. The spontaneous SCN neuronal firing rate patterns are similar to those observed in wild types. The Tau SCN firing rate rhythm displays a peak in neuronal activity occurring 5-6 h before the predicted onset of wheel-running activity, with a peak-to-peak period of 20 h. This period of the rhythm of spontaneous neuronal discharge within the Tau-mutant SCN parallels the behavioural free-running period of 20 h.