Ganciclovir (GCV) and its lipophilic elaidic acid ester prodrug E-GCV were evaluated for their antiherpetic, cytostatic and metabolic properties. E-GCV proved exquisitely inhibitory to the replication of herpes simplex virus type 1 (HSV-1) and HSV-2 in cell cultures (50% effective concentration (EC 50): 0.002 μM). It was five-to 10-fold more effective than its parent drug GCV. E-GCV was at least 2000-fold more cytostatic to HSV-1 or HSV-2 thymidine kinase (tk) gene-transfected mammary carcinoma FM3A tk−/HSVtk+ tumor cells than to the corresponding nontransfected tumor cells. The cytostatic activity of E-GCV to the HSVtk gene-transfected tumor cells was far superior to that of GCV. Metabolic studies revealed that both GCV and E-GCV were converted to the mono-, di-and tri-phosphate derivatives of GCV to a markedly higher extent in FM3Atk−/HSV-1 tk+ cells than in wild-type FM3A/0 cells. Strikingly, mono-, di-and tri-phosphate metabolites of GCV were retained for a substantially longer time in E-GCV-treated cells (half-life approximately 50 h) than in GCV-treated cells (half-life approximately 20 h). The longer retention time of the GCV metabolites most likely explains why E-GCV is superior to GCV against herpes simplex virus replication and HSVtk gene-transfected tumor cell proliferation. Taking into account the marked stability of E-GCV in human plasma and its much higher lipophilicity than GCV, E-GCV should be considered as an effective lipophilic prodrug of GCV with a markedly enhanced cytostatic activity in HSVtk gene-transfected tumor cells compared with parental ganciclovir. Its usefulness in the combined gene/chemotherapy of HSVtk gene-transfected tumors should be further pursued.