The carrier state has been confirmed virologically in Thoroughbred and non-Thoroughbred stallions naturally infected with equine arteritis virus (EAV). Short-term or convalescent and long-term carriers occur. The frequency rate of the long-term carrier state in Thoroughbreds was high, averaging 33.9% among the three groups of stallions under study. While the convalescent carrier state only lasted a few weeks after clinical recovery, the long-term carrier state could persist for years. There was evidence, however, that not all such carriers might remain persistently infected for life. Carrier stallions appeared to shed EAV constantly in the semen but not in the respiratory secretions or urine. It also could not be demonstrated in the buffy coat of the blood. All of the carrier stallions have continued to maintain moderate to high neutralizing antibody titres to EAV in serum. Virus was associated with the sperm-rich and not the pre-sperm fraction of semen. There was relatively little variation in virus concentration between sequential ejaculates from the same stallion. Transmission of EAV infection by long-term carrier stallions would appear to occur solely by the venereal route. Such carriers are thought to play an important epidemiological role in the dissemination and perpetuation of the virus.