The pathogenesis of turkey rhinotracheitis virus infection (TRTV) in 4 week-old chickens was studied by inoculation of a field strain via occular and nasal routes. This strain has been isolated from chickens with naturally occurring swollen head syndrome Respiratory signs were observed four days post-inoculation. Antibodies against TRTV infection were detected in the sera of the inoculated birds by agar gel diffusion test at the 9th day post-inoculation. The virus could be reisolated at the 7th day post-inoculation after three blind passages of the infected material in chicken embryos. Histopathology revealed serous rhinitis, sinusitis, catarrhal nasolacrimal adenitis, tracheitis, bronchitis and bronchiolitis. Scanning electron microscopy of the turbinates revealed focal deciliation and desquamation of the luminal surfaces of the ciliated epithelial cells at the early stages of infection. At ten days post-inoculation there were diffuse deciliation, epithelial cell exfoliation, epithelial hyperplasia with fusion of the hyperplastic folds at their luminal surface. The trachea and bronchi showed focal deciliation at the early stages of infection which became diffuse with obvious epithelial cell desquamation at the end of the experiment. Transmission electron microscopy further demonstrated the process of deciliation and exfoliation of the ciliated epithelial cells in the turbinates, trachea, bronchi and bronchioles. Ultrastructural changes involved the cilia and the cytoplasmic organelles of the ciliated cells. There were focal to diffuse deciliation, dilatation and vesiculation of the cisternae of the rough endoplasmic reticulum, and mitochondrial swelling Abnormalities of the cilia were in the form of swollen cilia, compound cilia and internalization of cilia. Although the virus could be reisolated on chicken embryos, trials for direct demonstration of the virus by transmission electron microscopy of the ciliated epithelial cells of the upper respiratory tract were unsuccessful.