The remarkably potent and specific activity against varicella-zoster virus (VZV) shown by 2'-deoxynucleosides of furo[2,3-d]pyrimidin-2(3H)-one and related ring systems is dependent on key structural features including the length and nature of the side-chain at C6 and the structure and stereochemistry of the sugar moiety at N3. Removal of the 3'-hydroxyl group from potent anti-VZV 2'-deoxynucleosides results in loss of the VZV activity, but such 2',3'-dideoxynucleoside analogues have shown anti-HCMV activity. We now report acyclic analogues with comparable side-chains at C6, but with the sugar moiety at N3 replaced with the (2-hydroxyethoxy)methyl group (present in the antiherpes drug acyclovir). Examples of both furo[2,3-d]- and pyrrolo[2,3-d]pyrimidin-2(3H)-one acyclic analogues were prepared and evaluated in a number of virus-infected cells and in tumor cell cultures. Certain of the long-chain analogues showed activity against VZV and HCMV. No significant activity against other DNA and RNA virus replication or against tumor cell proliferation was observed.