The role of the mammalian suprachiasmatic nuclei (SCN) in generating circadian rhythms in behaviours and other physiological processes is well established. A prominent feature of SCN neurons is the circadian oscillation in action potential firing frequency, with a peak near midday. A subset of calbindin-immunoreactive (CB+) neurons form a compact subnucleus (CBsn) in the hamster SCN. Restoration of rhythmicity using fetal SCN grafts in SCN-lesioned hamsters is critically dependent upon the presence of CB+ neurons within the transplanted grafts [LeSauter & Silver (1999) J. Neurosci., 5574-5585]. The aim of the current study was to determine whether CB+ neurons within the CBsn of the hamster SCN fire action potentials in a circadian pattern as part of their output signal. Using patch-clamp recording, we demonstrated that CB+ neurons in the CBsn do not express a circadian rhythm in spontaneous firing frequency under diurnal conditions in vitro. Furthermore, the percentage of silent CB- cells varies with zeitgeber time, whereas the percentage of silent CB+ cells does not. Immunohistochemical analysis revealed that the CBsn is a nonhomogeneous nucleus, containing many more CB- than CB+ cells. Our results reveal that CB+ neurons within the CBsn represent a functionally distinct neuronal subpopulation in which rhythmic action potential output may not be necessary for the restoration of behavioural circadian rhythmicity.