The ability of neurotropic alpha herpesviruses to replicate within synaptically linked neurons has made these pathogens valuable tools for transneuronal analysis. Recent studies suggest that unique gene products expressed by genetically engineered strains of virus may permit the use of multiple strains in complex tracing paradigms. In the present study we have examined the invasiveness of two genetically engineered strains of the swine pathogen known as pseudorabies virus(PRV). The two strains were isogenic with the attenuated Bartha strain of PRV; in one strain a lacZ reporter gene was inserted into the gC locus (PRV-BaBlu; $4.75 imes10^8pfu/ml$) contrained a PRV envelope glycoprotein gene that was absent in PRV-BaBlu. Simultaneous or temporally separated sequential injection of $4muell$ of each strain into the ventral wall of the stomach produced a predictale course of retrograde synaptic infection. The results were as follows: 1. PRV-BaBlu and PRV-D infected the dorsal motor nucleus of vagus nerve(DMV) and paraventricular nucleus(PVN). 2. Invasion and replication of PRV-D occured at a faster rate than the parental strain or PRV-BaBlu. 3. PRV-D was much more virulent than PRV-BaBlu or the parental strain. 4. Co-injection of PRV-D and PRV-BaBlu produced an infection that was more virulent than that produced by the parental strain (PRV-Bartha), 5. Neurons in DMV were permissive to co-infection with PRV-D and PRV-BaBlu when they were injected simultaneously into the same site. 6. Replication of PRV-BaBlu was compromised by prior infection of the same circuit with PRV-D. 7. Prior infection of neurons with PRV-D maked them resistant to infection with PRV-BaBlu.