We are working to understand the over-all organization of the circadian systems of vertebrates. Currently we have focused our attention on a transgenic rat model in which a  circadian clock gene (Per1) has been linked to a luciferase reporter, so that whenever the gene is expressed low levels of light are produced. Using sensitive photomultipliers we are able to track the circadian expression patterns of the Per1 gene in brain slices and in various cultured peripheral tissues. Thus we are able to ask and answer questions that have not previously been approachable such as: Do the clocks in all tissues remain in synchrony following a change in the timing of the light cycle to which the animals are exposed? What are the signals from the brain that influence the clocks in peripheral tissues? How do the temporal relationships among clocks in the brain and those in peripheral structures change during postnatal development? Because circadian rhythmicity is a fundamental property of virtually all living things, understanding of its basic mechanisms is certain to bring practical benefits.