Abstract: Peganum harmala L. (Zygophylaceae) has been used in a variety of practical applications in medical science. Our objective in the current study was to determine the effects of the seed extract on Herpes simplex virus type 1 (HSV-1) replication in Vero cells. Different concentrations (100, 500, 571, 667, 800 and 1000 µg/ml) of the extract were examined. Experiments were carried out using Vero cells. P.harmala seed extract was found to be non-toxic to Vero cells up to a concentration of 667 µg/ml. The antiviral activity of non-toxic concentration against HSV-1 was tested. The replication of HSV-1 was inhibited, indicating that the P.harmala L. extract contains an anti-HSV-1 substance.