Histone deacetylase (HDAC) inhibitors represent a promising new therapy against malignant glioma. When used in conjunction with oncolytic viral vectors, these compounds have been shown to augment virotherapy. In the current study, we examined the antitumor effect of combining the lytic animal virus equine herpesvirus type 1 (EHV-1) with the HDAC inhibitor valproic acid (VPA). Pretreatment of two human glioblastoma cell lines (U251 and SNB19) with VPA resulted in a significant increase in virus entry, replication, cell to cell spread and cell lysis. Overall, these data indicate that VPA significantly improves EHV-1-mediated oncolysis of human glioma cells that are only moderately killed by EHV-1 alone.