Cellular, physiological and behavioural functions of many organisms exhibit approximate 24 hour periodicity. This periodicity in mammals is driven and coordinated by the master circadian pacemaker, located in the suprachiasmatic nuclei (SCN). Neurons here exhibit cell autonomous cycling of core clock genes and proteins. More recent evidence demonstrates rhythmic expression of core clock genes and proteins in extra-SCN brain areas and in many peripheral tissues, revealing the circadian system as a network of peripheral and neuronal oscillators, synchronised by the SCN. Mammalian circadian organisation diminishes with age and this altered circadian function is increasingly recognised as a risk factor in health and well-being. Although in general, circadian rhythms are reported to be disrupted in aged rodents, there are inconsistencies, with reports of shortening, lengthening and no effects on free running period of physiological and behavioural rhythms. To clarify this, we assessed the effects of ageing on wheel-running activity and clock gene rhythms in mice. Female mPer2Luc knock-in (PER2::LUC) mice were group-housed under a 12-h light:12-h dark (12:12 LD) cycle from birth. Wheel running activity was assessed in singly housed 2-4 and 19-22 month old mice, firstly in a 12:12 LD cycle for 2 weeks, then in constant dark conditions for 2 weeks, followed by transfer back to the previous LD cycle. Subsequently, we recorded PER2::LUC expression in central (SCN, arcuate nucleus, dorsomedial hypothalamus, pituitary), and in peripheral (heart, lung, liver, adrenal) tissues from young and aged mice. Coronal brain slice sections (250μm) prepared with a vibroslicer, hand sliced peripheral tissue sections (~1mm) and whole pituitary and adrenal glands were cultured individually, and PER2::LUC bioluminescence emission was monitored at 37°C in the dark with photon multiplier tube assemblies. Further, PER2::LUC expression was visualised in individual cells in SCN slice cultures imaged with a CCD camera every 1 hour for a period of 7 days. There was no significant difference in circadian periodicity in the heart and lung of young and old mice (p> 0.05; t-test). In contrast, the period of the adrenal rhythm calculated in aged (mean 20.9 ± 0.58h SEM) was shortened significantly compared with young (23.7 ± 0.39h; p<0.05) mice. We also report an age-related decline in the amplitude of the SCN (Relative bioluminescence in young: 22632 ± 3280; in old: 10831 ± 2468; p<0.05) and arcuate nucleus (Relative bioluminescence in young: 648 ± 97; in old: 286 ± 59; p<0.05). The current data highlight differential effects of ageing on central and peripheral oscillators throughout the circadian system.