Abstract. Adenoviruses and reoviruses isolated from commercial broiler chickens were evaluated for gastrointestinal pathogenicity in specific-pathogen-free Leghorn chickens. The viruses were originally isolated from either the proventriculus or a gastrointestinal pool of tissues of broiler chickens with proventriculitis or enteritis. Isolates were cloned by terminal dilution. Day-old chickens were inoculated by oral and ocular routes with undiluted tissue culture fluids (titers of 10 2 -10 4 TCID 50 /ml) and then examined at necropsy on days 5, 10, and 15 postinoculation. Chickens in all virus groups (but not the control group) developed wet, unformed fecal droppings that persisted for the duration of the study. Mild lesions occurred in reovirus-inoculated chickens and included hyperplasia of lymphocyte aggregates in various organs and mild gizzard erosions. Chickens inoculated with adenovirus isolates developed marked gizzard erosions and necrotizing pancreatitis as well as mild proventriculitis. Intranuclear viral inclusion bodies occurred in gizzard epithelium and pancreatic acinar cells at the sites of lesions. Lymphocytic atrophy occurred in the bursa of Fabricius. Respective viruses were reisolated from proventriculus and duodenum collected from chickens of each group; no viruses were isolated from controls. Under the conditions of this study, adenovirus isolates were more pathogenic than the reovirus isolates in the digestive system.Digestive system signs and lesions often are observed in broiler chickens submitted for laboratory evaluation because of impaired weight gain and feed conversion, feed passage, and watery feces. Various lesions have been identified in the proventriculus, gizzard, intestines, pancreas, and liver in the course of these investigations. Adenoviruses and reoviruses are commonly isolated from the affected viscera, particularly from the proventriculus and duodenum in birds with lesions in these organs.Digestive tract disease in broiler chickens in the southeastern United States has several causes and clinical and pathologic expressions. 7 Reoviruses have a causative link with digestive syndromes, but mainly these syndromes are caused by certain reovirus strains of high pathogenicity. 23 The role of adenoviruses in digestive disease of chickens is equivocal, 18 which has made it difficult to interpret virus isolation results from chickens with digestive system signs and lesions.The purpose of this study was to extend routine diagnostic investigation of digestive tract disease by attempting to assess the pathogenic potential of viral isolates in susceptible chickens.