The pathogenesis of inclusion body hepatitis was studied following the oral administration of a serotype 8 strain of avian adenovirus into 2-day-old specific pathogen free chickens. Viral antigens were detected in tissues at various times post inoculation (pi) by enzyme-linked immunosorbent assay and by immunocytochemistry. Viral antigens were detected in intestinal epithelium from 12h to 13 days pi and in the plasma fraction of blood by 24 h pi. A biphasic, cell-free viremia with peaks at 2 and 7 days pi was recorded. Antigens were first detected in the liver from 2 days and reached peak levels at 6 days pi. The second peak of viral antigens in blood plasma was probably due to release of virus from damaged hepatic cells. Initially, viral antigens in the liver were restricted to cells lining the sinusoids but increasing involvement of hepatocytes occurred with time. Small amounts of viral antigens were detected in other tissues. Following the appearance of neutralizing antibodies in serum from 7 days pi, the levels of viral antigens in all tissues decreased and were undetectable by 15 days pi. This viral hepatitis of chickens is possibly a useful model for other viral infections where a cell-free viremic phase is important for spread of virus from primary sites to target organs, such as the liver.