Herpesviruses are known to influence expression of major histocompatibility complex (MHC) class I molecules on the surface of infected cells using a variety of mechanisms. Downregulation of MHC class I expression prohibits detection and elimination of infected cells by cytotoxic T lymphocytes. To investigate the effect of rat cytomegalovirus (RCMV) infection on MHC class I expression, we infected immortalized and primary rat fibroblasts with RCMV and monitored surface expression of MHC class I molecules at various time-points postinfection. These experiments revealed a downregulation of MHC class I surface expression by RCMV, a phenomenon that has also been reported for human and murine CMV. However, in contrast to the other cytomegaloviruses, RCMV causes only a temporal downregulation of MHC class I, with a maximal decrease at 12 h postinfection. Unlike murine and human CMV, RCMV does not induce proteolytic degradation of MHC class I molecules. In RCMV-infected cells, the MHC class I molecules are stable, but their exit from the ER is delayed.