The ultrastructure of the proximal tubular epithelial cells in chicken kidneys was examined throughout the course of an experimental infection with infectious bronchitis virus. A quantitative assessment of the structural changes in the cells was related to these in normal cells. Significant alterations were detected in the membrane structures and in the mitochondria. There was a reduction in surface area of the microvillus membrane, the basolateral membrane and the apical tubular membrane. There were alterations in the shape of mitochondrial profiles and a decrease in the volume density of mitochondria. Vesicular structures, which are a possible site of viral release, were observed in the lateral surface of cells. These changes in the functional components of the cells indicate impaired transport of ions and water. The results demonstrate the value of stereological methods for the study of viral-host cell interactions in the pathogenesis of viral disease.