A number of 4-substituted 7-(ethoxymethyl)- and 7-[(2-methoxyethoxy)methyl]pyrrolo[2,3-d]-pyrimidine-5-carbonitrile and -5-thiocarboxamide derivatives and several 7-substituted 4,6-diaminopyrrolo[2,3-d]pyrimidine-5-carbonitrile, -5-carboxamide, and -5-thiocarboxamide analogs related to the nucleoside antibiotics toyocamycin and sangivamycin were prepared and tested for activity against human cytomegalovirus (HCMV) and herpes simplex virus type 1 (HSV-1). Biologically, modifications at the 4-position were not well tolerated in cell culture, and in almost all cases no activity against HCMV or HSV-1 was observed. Furthermore, none of the compounds inhibited the growth of L1210 murine leukemic cells in vitro. In sharp contrast to the 4-substituted compounds, all of the 4,6-diamino 5-nitrile and the 5-thioamide analogs were active against HCMV, whereas the 5-carboxamides were inactive. The corresponding 4-amino 6-methylamino and 6-dimethylamino 5-nitrile analogs were inactive against HCMV, establishing that an amino group at both C-4 and C-6 is a likely requirement for antiviral activity. Overall, our results demonstrate that an amino group at C-4 and a thioamide moiety at C-5 of a 7-substituted pyrrolo[2,3-d]pyrimidine are essential for activity against HCMV, whereas a 4,6-diamino analog does not necessarily require a thioamide group at C-5 for activity against HCMV.