Background: Sodium valproate (VPA), an anticonvalsant drug, has been reported to stimulate viral replication. A combination therapy with VPA and acyclovir (ACV) is used for the treatment of herpesvirus encephalitis, the commonest sporadic encephalitis of viral origin. Objective: To determine a possible interaction between VPA and ACV leading to a modification of antiviral activity of ACV. Methods: Cultured Hela cells were treated with 5mM of ACV and various concentrations of VPA followed by infection with herpes simplex virus type 1 (HSV-1). Virus replication was monitored by quantal assay. Further investigations comprised electron microscopy, immunoperoxidase and immunoblot procedures. Possible chemical interaction between VPA and ACV was studied by nuclear magnetic resonance (NMR) spectrometer. Results: Combined treatment of infected cells with ACV and VPA revealed 50- to 250-fold potentiation of antiviral activity of ACV by increasing VPA concentrations. Examination by NMR spectrometer showed a strong chemical interaction between amino groups of ACV and carboxyl part of VPA. Conclusion: The present in vitro studies should be paralleled by appropriate in vivo investigations, and if substantiated, a combination therapy with ACV and VPA may supersede single ACV therapy for herpesvirus encephalitis. Further studies are thus needed to establish which of VPA metabolites or newly-formed compounds is accountable for augmentation of antiviral effect of ACV.