The suprachiasmatic nuclei of the hypothalamus (SCN) are the master circadian clock in mammals. Transcriptional activity in this master clock has a marker in the immediate-early gene c-Fos. Within the SCN, distinct differences in c-Fos in the ventrolateral and the dorsomedial SCN have been reported for rodent species such as rats, mice, and hamsters. We studied C-FOS expression in the common vole (Microtus arvalis) SCN under LD 12:12 h and under constant dim light conditions. In the vole dorsomedial SCN, rhythmic C-FOS expression was seen in LD with a clear peak in the middle of the light period. Under constant dim light, we report constitutive, non-rhythmic expression of C-FOS in the dorsomedial SCN. This pattern is consistent with the circadian organization of behavioral activity, which is weak in voles and may be lost under constant dim-light conditions. In the ventrolateral SCN, we observed a rise in C-FOS expression under LD conditions prior to lights-on, followed by peak expression at lights-on. Another peak was seen at lights-off. In an additional experiment, we subjected animals to LD 16:8 to test the hypothesis that the dawn and dusk peaks in ventrolateral C-FOS expression change phase along with the photoperiod. The peak in C-FOS expression did not shift with the time of lights on, but remained at the same external time 6. The results are consistent with the interpretation that in the vole, c-Fos expression reports transcriptional activity associated more likely with an internal, gating process than with an external effect of light.