Four calves (control animals) were inoculated (by aerosol) with Cooper's strain of infectious bovine rhinotracheitis (IBR) virus given with an ultrasonic nebulizer. Seven other calves (principals) were inoculated (by aerosol) with a noncytopathic strain of bovine viral diarrhea (BVD) virus and 7 days later, with IBR virus. The calves were killed 3 to 8 days after the latter virus was inoculated. Various tissues were collected from the calves during necropsy and assayed for IBR virus. In the control calves, the IBR virus was recovered only from the cranial part of the respiratory tract and, in much lower concentrations, from the caudal part of the respiratory tract. In contrast, the same virus was widely disseminated in most tissues, usually in high concentrations, of the calves first inoculated with BVD virus (principals). These observations indicate that initial BVD virus infection may impair the ability of calves to clear IBR virus from the lungs and to contain the latter virus at the local infection site. The pathogenesis of various forms of IBR virus-induced diseases in calves is discussed.