The synthesis of potential "combined prodrugs" wherein phosphonoformate or phosphonoacetate was attached to the 5'-position of 2'-deoxyuridine, 2'-deoxythymidine, 5-iodo-2'-deoxyuridine (IDU), 5-(2-chloroethyl)-2'-deoxyuridine (CEDU), or 5-(2-bromovinyl)-2'-deoxyuridine (BVDU) or to the 3'-position of CEDU is described. The antiviral activities of these derivatives and of reference compounds were compared in Vero, HEp-2, and primary rabbit kidney cells against herpes simplex virus types 1 and 2 (HSV-1 and -2). The CEDU and BVDU analogues were also evaluated against systemic and intracutaneous HSV-1 infection in mice. The nature of the 5-substituent proved critical for antiviral activity, since only the 5-iodo-, 5-(2-bromovinyl)-, and 5-(2-chloroethyl)-substituted derivatives were inhibitory to the herpesviruses. Furthermore, the type specificity is determined by the nature of the 5-substituent: the IDU analogues were similarly inhibitory to HSV-1 and -2 whereas the CEDU and BVDU analogues inhibited HSV-2 replication only at considerably higher concentrations than HSV-1. In vivo, several derivatives were shown to possess significant antiviral activity; however, none surpassed its respective parent compound, CEDU or BVDU, in potency. It seems improbable, therefore, that a synergistic effect between PFA or PAA and the nucleoside analogue occurred. The extent of in vitro and in vivo activity of the CEDU and BVDU 5'-phosphonoformates and 5'-phosphonoacetates is most plausibly explained by the ease by which the "combined prodrugs" are hydrolyzed and the parent compound, CEDU and BVDU, respectively, is released.