Sleep is understood to possess recuperative properties and, conversely, sleep loss is associated with disease and shortened lifespan. Despite these critical attributes, the mechanisms and functions by which sleep amount impacts health still are speculative. One of the most consistent, if largely overlooked, signs of sleep loss in both humans and laboratory rats is a progressive increase in circulating phagocytic cells, mainly neutrophils. The destination, if any, of the increased circulating populations has been unknown and, therefore, its medical significance has been uncertain. The purpose of the present experiment was to determine the content and location of neutrophils in liver and lung tissue of sleep-deprived rats. These are two principal sites affected by neutrophil migration during systemic inflammatory illness. The content of neutrophils in the intestine also was determined. Sleep deprivation in rats was produced for 5 and 10 days by the Bergmann-Rechtschaffen disk method, which has been validated for its high selectivity under freely moving conditions and which was tolerated and accompanied by a deep negative energy balance. Comparison groups included basal conditions and 48 hrs of sleep recovery after 10 days of sleep loss. Myeloperoxidase (MPO), an enzyme constituent of neutrophils, was extracted from liver, lung, and intestinal tissues, and its activity determined by spectrophotometry. Leukocytes were located in vasculature and interstitial spaces in the liver and the lung by immunohistochemistry. Heme oxygenase-1, also known as heat shock protein-32 and a marker of cellular stress, and corticosterone also were measured. The results indicate neutrophil migration into extravascular liver and lung tissue concurrent with cell stress and consistent with tissue injury or infection induced by sleep loss. Plasma corticosterone was unchanged. Recovery sleep was marked by increased lung heme oxygenase-1, increased intestinal MPO activity, and abnormally low corticosterone, suggesting ongoing reactive processes as a result of prior sleep deprivation.