The effect of substitution in the tricyclic moiety of 3,9-dihydro-9-oxo-5H-imidazo[1,2-alpha]purine (1,N-2-ethenoguanine) analogues of acyclovir (1) and ganciclovir (2) on their physical properties and antiherpetic activity was investigated by synthesizing a series of compounds substituted in the 2, 6, or 7 position (6-14). Substitution in the 6-position with phenyl or 4-biphenylyl resulted in fluorescent compounds (7, 9, 13, 14). In general, the substituent in the 6 position potentiated the antiviral activity. The fluorescent 6-phenyl derivatives: 3,9-dihydro-3-[(2-hydroxyethoxy)methyl]-9-oxo-6-phenyl-5H-imidaxo[ 1,2-alpha]purine (7) and its 3-[(1,3-dihydroxy-2-propoxy)methyl] congener (13) were the most potent tricyclic analogues of 1 and 2, respectively. Compound 7 was inhibitory to TK+ HSV-1, TK+ HSV-2, and TK+ VZV within the concentration range of 0.2-2.0 micrograms/mL, well below the cytotoxicity threshold (50 to > 100 micrograms/mL). Compound 13 was inhibitory to TK+ HSV-1 and TK+ HSV-2 within the concentration range of 0.005-0.3 microgram/mL and to TK+ and TK- VZV within the concentration range of 0.4-3 micrograms/mL (cytotoxicity threshold > 200 micrograms/mL). Both 7 and 13 seem to be promising candidate compounds for the noninvasive diagnosis of herpesvirus infections.