Tenofovir [9-(R)-2-(phosphonomethoxypropyl)adenine (PMPA)] and zidovudine [azidothymidine (AZT)] are potent anti-HIV agents that have shown a strong synergy in in vitro studies. In this paper we have investigated both the potentiality of this synergy in vivo and the possibility to administer AZT and PMPA simultaneously as a single drug AZTpPMPA. The pharmacokinetic studies reported here have shown that AZTpPMPA administered intraperitoneally in mice performs as a prodrug, providing a slow delivery of AZT and PMPA in circulation. C57BL/6 mice infected with the retroviral complex LP-BM5 were used to evaluate the efficacy of AZTpPMPA in inhibiting disease progression. Furthermore, the effectiveness of the heterodinucleotide was compared with that of AZT and PMPA, administered as single drugs, or as a combination (AZT plus PMPA). The results obtained showed that AZTpPMPA is able to reduce lymphoadenopathy (88%), splenomegaly (64%), lymph node BM5 proviral DNA content (49%) and hypergammaglobulinaemia (40%). However, upon AZT plus PMPA administration, similar (splenomegaly and lymphoadenopathy reduction) or better results (64% hypergammaglobulinaemia reduction and 75% lymph node BM5 proviral DNA content inhibition) were obtained. Furthermore, these results overlapped those obtained upon PMPA administration. Thus, no synergy between PMPA and AZT was observed in murine AIDS and administration of AZT does not improve the antiviral results obtained by PMPA administration.