Diurnal is a web-based tool for accessing the diurnal and circadian genome-wide expression results of genes from several array experiments conducted on common model plants. The expression patterns of particular loci or array probes can be looked up under a variety of different conditions. For each condition, every expression pattern has been modeled using a periodic pattern, which has then been used to estimate the phase of peak expression. These results can be accessed both graphically online and downloaded to your computer for further use.