BACKGROUND Using canine distemper virus (CDV) encephalomyelitis as a model for morbillivirus-induced intranuclear inclusion body (INB) formation, we previously demonstrated that the cellular basis for INB within infected astrocytes are viral antigen-associated nucleolar derivatives known as complex nuclear bodies. Here, we examined the relationship between the cellular stress response and intranuclear CDV protein trafficking so that we may begin to define cellular events leading to formation of complex nuclear bodies containing viral protein. EXPERIMENTAL DESIGN Confocal dual-label immunofluorescence microscopy was used to simultaneously determine the distribution of CDV antigen and the major inducible 70 kilodalton (kd) heat shock protein (i.e., 72 kd heat shock protein (HSP)) within infected canine central nervous tissue. This dual-label immunofluorescent technique was also used to test the relationship between cellular stress and CDV core protein distribution in a CDV persistently infected cell line. RESULTS The cytoplasm of INB-positive astrocytes expressed elevated 72 kd HSP characteristic of the cellular stress response; INB contained 72 kd HSP and the major CDV nucleocapsid protein (i.e., N). Intranuclear inclusion bodies did not stain positively for the CDV nucleocapsid phosphoprotein nor was intranuclear N protein observed in the absence of nuclear or elevated cytoplasmic 72 kd HSP. In the persistently infected cell line, induction of the cellular stress response elevated 72 kd HSP and produced translocation of the CDV N protein to the nucleus from its normal location in the cytoplasm of unstressed infected cells. As observed in vivo, the transport was specific to N and was correlated to the induction of complex nuclear bodies. The latter was documented by transmission electron microscopy, where complex nuclear bodies formed in shocked infected cells, but not shocked uninfected cells or nonshocked infected cells. CONCLUSIONS CDV infection in vivo induces the cellular stress response. Paradoxically, this normally protective cellular response mediates redistribution of viral N protein from the cytoplasm into infected cell nuclei, which is the likely basis for complex nuclear body formation and a unique form of virus-induced cytopathic effect.