Syrian hamsters display age-related changes in the expression of circadian rhythms and in responsiveness of the circadian system to photic and non-photic stimuli. This study characterized the effects of age on the locomotor activity rhythm of middle-aged and old hamsters and evaluated the effects of strengthening the entraining light signal. Compared with young (4.5 mo) animals, middle-aged (11.25 mo) and old (16 mo) animals displayed increased daily bouts of activity (P < 0.001) and reduced total daily activity and activity rhythm amplitude (P < 0.05) in 14:10-h light-dark cycles. After the light intensity was increased from 300 to 1,500 lx during the light cycle, middle-aged hamsters demonstrated decreased daily activity bouts (P < 0.05) and increased total daily activity (P < or = 0.01) and activity rhythm amplitude (P < or = 0.001) compared with controls maintained in 300 lx. The pattern of changes in the activity rhythm of old experimental animals was similar to trends observed in middle-aged experimental hamsters, although not as robust. Thus age-related changes in the activity rhythm are occurring by middle age in hamsters, and the provision of stronger entraining signals may lead to more stable circadian organization.