Hartley guinea pigs infected with guinea pig cytomegalovirus (GP-CMV) develop a mononucleosis syndrome during primary infection that is similar to that seen in immunocompetent humans. In the present study this animal model of human disease was used to investigate the sequential changes in the lymph nodes during the course of primary GP-CMV infection. Infectious virus could be isolated from the nodes up to eight weeks after inoculation. When compared with nodes of control animals, the nodes of GP-CMV-infected animals were found to be enlarged up to a year after infection. During the first ten days of infection, histologic changes due to virus proliferation and immune stimulation of the paracortical areas of the lymph node, in a diffuse hyperplasia pattern, were noted. Although typical cytomegalovirus inclusions were seen only rarely, many cells demonstrated intranuclear staining using an avidin-biotin immunoglucose-oxidase histochemical reaction for GP-CMV. During late acute and chronic infection, the lymph nodes showed immune stimulation of the germinal centers, in the pattern of follicular hyperplasia. Specific histologic changes related to active virus proliferation were not seen after the early acute phase.