a new series of 6-substituted N1- and N2 -β-D-ribofuranosyl-pyrazolo[3,4-d]pyridazin-7(6H)-one derivatives (8a-g, 9b-d and 9f-g) has been prepared in the aim to explore the structure-activity relationships and improve the antiviral activity of a series of N1-and N2 -β-D-ribofuranosyl-pyrazolo[4,3-d]pyrimidin-7(6H)-one derivatives previously reported by us. The studied compounds were preliminary tested for their in vitroa antiviral activity against HSV-1. Among them 8f and 9f were found 2 fold more active than those belonging to the parent series.