Abstract. Venezuelan equine encephalitis (VEE) viruses cause natural outbreaks in humans and horses and represent a significant biothreat agent. The effect of tunicamycin on the course of the disease in mice with VEE was investigated, and the combined effects of these agents was characterized. CD-1 mice given 2.5 mg of tunicamycin had .1,000-fold more virus in the brain 48 hours after infection with the virulent VEE strain V3000 and $100-fold of the attenuated strain V3034 at all tested times than did untreated mice, indicating enhanced neuroinvasion. Tunicamycin did not alter the viremia profiles of these viruses nor the replication of V3000 in the brain itself. Tunicamycin alone caused ultrastructural blood-brain barrier damage, yet neuroinvasion by V3000 in treated mice appeared to occur via the olfactory system rather than the blood-brain barrier. Tunicamycin-treated, V3000-infected mice also exhibited earlier and more severe weight loss, neurological signs, neuronal infection, neuronal necrosis and apoptosis, and inflammation than untreated, V3000-infected mice. The mean survival time of tunicamycin-treated, V3000-infected mice was 7.3 days versus 9.9 days for untreated, V3000-infected mice. These studies imply that animals that ingest toxins similar to tunicamycin, including the agent of annual ryegrass toxicity in livestock, are conceivably at greater risk from infections by encephalitis viruses and that humans and horses exposed to agents acting similar to tunicamycin may be more susceptible to encephalitis caused by VEE viruses. The exact mechanism of tunicamycin-enhanced neuroinvasion by VEE viruses requires further study.