The nuclear changes occurring in dog kidney cells infected with a virulent and an attenuated strain of canine distemper virus were examined by electron microscopy. No ultrastructural evidence for intranuclear virus replication was observed even though large eosinophilic nuclear inclusions were found in up to 34% of infected cells examined by light microscopy. This discrepancy between light and electron microscopic findings was discussed in relation to the possible structure of the nuclear inclusions. Marked morphological changes were noted in the nucleoli of up to 12% of cells infected with the virulent strain, CDV/BR, involving complete segregation of the granular and fibrillar components. The nucleolar lesions were similar to those induced by chemical inhibitors of cellular nucleic acid metabolism.