The circadian expression patterns of genes encoding for proteins that make up the core of the circadian clock were measured in rat retina using real-time quantitative PCR (qPCR). Transcript levels of several genes previously used for normalization of qPCR assays were determined and the effect of ischemia-reperfusion on the expression of clock genes was studied. Statistically significant circadian changes in transcript levels were found for: Per2, Per3, Cry2, Bmall, Rora, Rorb, and Rorc with changes ranging between 1.6- and 2.6-fold. No changes were found for Perl, Cryl, Clock, Rev-erb alpha, and Rev-erb beta. Significant differences in transcript levels were observed for several candidate reference genes: HPRT, GAPDH, rhodopsin, and Thyl and, consequently, the use of these genes for normalization purposes in qPCR or Northern blots may lead to erroneous conclusions. Ischemia-reperfusion leads to a persistent decrease of Perl and Cry2, which may be related to the selective degeneration of amacrine and ganglion cells. We conclude that while all clock Genes are expressed in the retina, only a few show a clear circadian pattern. (c) 2005 Elsevier Inc. All rights reserved.