Adenoviral vectors were inoculated via intracardiac injection into 5- to 1O-week-old cotton rats (Sigmodon hispidus) to evaluate the effects of systemic delivery. Cotton rats were chosen as a model because they are semipermissive to the replication of human adenoviruses. The vector used was AdV.RSV-tk, a replication-deficient adenovirus with a herpes simplex virus thymidine kinase gene inserted in the E1 region. Vector doses were 3 x 10(8), 3 x 10(9), and 3 x 10(10) viral particles per animal with and without ganciclovir at 10 mg/kg twice a day. Animals were sacrificed and necropsied at 24 hours, 7 days, and 14 days postinoculation. Gross and microscopic pathologic observations in dosed groups were compared with an unmanipulated control group. From each animal, 10 different organ systems were analyzed for histopathology and vector distribution. The only significant microscopic lesions observed were epicardial inflammation and splenic hemosiderosis. Vector sequences persisted throughout the 14-day assay with preponderance in the heart, lung, and lymphoid organs. Infectious virions were detected for 24 hours, and these virions were only detected at the site of injection of two animals in the highest dose group. No viral replication was detected. Therefore, systemic delivery of up to 3 x 10(11) viral particles/kg was well tolerated in this semipermissive host model and did not result in any significant pathology.