The effects of aging on neuronal activity in the suprachiasmatic nucleus (SCN) were examined in hamsters kept under light-dark (LD) or constant light (LL) conditions. The free-running period in wheel-running rhythm of 24-month-old hamsters (24.2 +/- 0.04) was shorter than that of the 2-month-old hamsters (24.4 +/- 0.057). There was a significant difference in the mean firing rates of SCN neuron activity between old and young hamsters during subjective day (6.58 +/- 0.36 spikes/s in young and 5.63 +/- 0.24 in old hamsters), but not during subjective night (4.33 +/- 0.47 in young and 4.05 +/- 0.39 in old). Similar to LL condition, the firing activity during zeitgeber time 3-11 (4.33 +/- 0.27) in old hamsters kept under LD condition, was significantly lower than that of young hamsters (6.22 +/- 0.32). These results suggest that deterioration of SCN neuronal activity in old hamsters assessed as reduction of daytime activity may reflect changes in the interaction between SCN clocks and the overt behaviors and/or pacemaking properties of SCN cells.