ABSTRACT: Circadian rhythms are oscillations with a periodicity of 24 hours that are controlled by an endogenous clock and are observed in virtually all aspects of mammalian function from expression of genes to complex physiological processes. The master clock is present in the suprachiasmatic nucleus (SCN) in the anterior part of the hypothalamus and controls peripheral clocks present in other parts of the body . Although much is known about the mechanism of the central clock in the SCN, the regulation of clocks present in peripheral tissues is still unclear. This study is designed to examine fluctuations in gene expression in lungs within the 24 hour circadian cycle in normal animals. The objectives of this study is to identify and analyze circadian oscillation in gene expression in lungs, and to identify the role of circadian regulation in coordinating the functioning of this dynamic organ. Fifty-four male normal Wistar rats (250-350 g body weight) were housed in a strictly controlled stress free environment with light:dark cycles of 12 hr:12hr. Three animals were sacrificed at each of 18 selected time points within the 24 hour cycle. RNA was prepared from lungs for gene arrays. Time point designations reflect time after lights on in hours.