Venezuelan equine encephalitis virus (VEEV) is an important, naturally emerging zoonotic pathogen. Recent outbreaks in Venezuela and Colombia in 1995, involving an estimated 100,000 human cases, indicate that VEEV still poses a serious public health threat. To develop a safe, efficient vaccine that protects against disease resulting from VEEV infection, we generated chimeric Sindbis (SIN) viruses expressing structural proteins of different strains of VEEV and analyzed their replication in vitro and in vivo, as well as the characteristics of the induced immune responses. None of the chimeric SIN/VEE viruses caused any detectable disease in adult mice after either intracerebral (i.c.) or subcutaneous (s.c.) inoculation, and all chimeras were more attenuated than the vaccine strain, VEEV TC83, in 6-day-old mice after i.c. infection. All vaccinated mice were protected against lethal encephalitis following i.c., s.c., or intranasal (i.n.) challenge with the virulent VEEV ZPC738 strain (ZPC738). In spite of the absence of clinical encephalitis in vaccinated mice challenged with ZPC738 via i.n. or i.c. route, we regularly detected high levels of infectious challenge virus in the central nervous system (CNS). However, infectious virus was undetectable in the brains of all immunized animals at 28 days after challenge. Hamsters vaccinated with chimeric SIN/VEE viruses were also protected against s.c. challenge with ZPC738. Taken together, our findings suggest that these chimeric SIN/VEE viruses are safe and efficacious in adult mice and hamsters and are potentially useful as VEEV vaccines. In addition, immunized animals provide a useful model for studying the mechanisms of the anti-VEEV neuroinflammatory response, leading to the reduction of viral titers in the CNS and survival of animals.Venezuelan equine encephalitis virus (VEEV) is an enveloped virus with a nonsegmented, positive-sense RNA genome of approximately 11.4 kb and belongs to the Alphavirus genus in the Togaviridae family. The 5Ј two-thirds of the genome contains four nonstructural proteins (nsP1 to nsP4) that form an enzyme complex required for viral replication (46-48). After release of the viral genome into the cytoplasm, a nonstructural polyprotein is translated directly from this RNA and utilized in the production of a full-length, negative-sense replicative RNA intermediate (45). The full-length RNA then serves as a template for the synthesis of positive-sense genomic RNA and for transcription of a subgenomic 26S RNA (46). The approximately 4-kb-long, subgenomic RNA corresponds to the 3Ј onethird of the viral genome and is translated into a structural polyprotein that is proteolytically cleaved into the capsid and the envelope glycoproteins E2 and E1 (34). Two hundred forty copies of the capsid protein enclose the genomic viral RNA to form an icosahedral nucleocapsid that buds from the plasma membrane, acquiring a lipid envelope with embedded protein spikes formed by E1/E2 heterodimers (41, 48).Venezuelan equine encephalitis virus is a zoonotic pathogen...