Circadian rhythms describe behavioural and physiological processes that oscillate over the 24-hour day. The suprachiasmatic nucleus (SCN) is the primary pacemaker located in the anterior hypothalamus of mammals and is necessary for driving circadian rhythms. As the SCN receives direct light input from the eye, light can be used to shift the phase of circadian rhythms, such as phase of activity onset. Recently, our lab found extracellular matrix structures called perineuronal nets (PNNs) in the SCN of Syrian hamsters (Mesocricetus auratus), and pilot studies are suggesting that they may play a functional role in the photic response of the circadian system. As there is no known literature describing PNNs in the SCN of any model rodents, the work in this thesis explored the expression of PNNs in the SCN of hamsters, Sprague-Dawley rats (Rattus norvegicus) and C57BL/6 mice (Mus musculus), using staining with Wisteria floribunda agglutinin (WFA). Further investigations were performed on hamsters to determine the cell types that PNNs associated with in the hamster SCN, and to determine if PNN removal would change phase delays and c-Fos expression in response to a light pulse at circadian time 13. We hypothesized that PNN removal would increase the delay response as well as increase the light-induced expression of c-Fos in the SCN. Our findings show that mice do not show WFA staining in the SCN, while rats show light staining in the central SCN core and hamsters show staining through the whole SCN. In the hamsters, PNNs seem to associate most highly with AVP and CalB, with a line analysis in the SCN core showing greater PNN intensity over the CalB region but an ROD analysis showing a nonsignificant difference between CalB, VIP and GRP regions. Finally, PNN removal increased the hamsters’ response to a phase delaying light pulse, but did not increase the c-Fos response. The work from this thesis shows that there are vast species differences in PNN expression in the SCN and helps support the hypothesis that PNNs may help limit the photic response of the circadian system.