Porcine endogenous retrovirus (PERV) may potentially be transmitted through porcine xenotransplantation products administered to humans. This study examined the feasibility of using guinea pigs as a model to characterize the in vivo infectivity of PERV. To enhance the susceptibility of guinea pigs to retroviral infection or genomic integration, moderate physiological or immunological changes were induced prior to exposing the animals to PERV. Quantitative PERV-specific PCR performed on all tested samples resulted in either undetectable or very low copy numbers of proviruses, even in animals possessing PERV-specific antibody responses. The low copy number of viral DNA detected suggests that PERV infected a limited number of cells. However, PERV DNA levels did not increase over time, suggesting no virus replication occurred. These results in the guinea pig are similar to previous observations of non-human primate cells that allow PERV infection but do not support PERV replication in vitro.