Human immunodeficiency virus (HIV) can be transmitted in semen from a man to his sexual partners. Antiretroviral drugs are likely to affect the amount of HIV-1 in semen and perhaps transmission of the virus. The concentrations of zidovudine, lamivudine, and HIV-1 RNA in blood and seminal plasma were measured in 9 HIV-positive men over </=2 years. Median (25th-75th percentiles) zidovudine blood and seminal plasma concentrations were 64.2 (range, 48.4-206.9; n=82) and 292.5 (range, 194.3-438.4; n=79) ng/mL, respectively. Median lamivudine blood and seminal plasma concentrations were 391.3 (range, 175.3-793.8; n=82) and 2701.8 (range, 1460.5-4320.2; n=79) ng/mL, respectively. The concentration of HIV-1 RNA in seminal plasma was monitored as a potential surrogate marker for infectiousness. RNA became undetectable (<400 copies/mL) in the blood and seminal plasma of 8/9 subjects after initiation of therapy and remained undetectable in 6/9 subjects. These data show that zidovudine and lamivudine achieve high concentrations in seminal plasma and significantly reduce HIV-1 RNA. The effects of antiviral therapy on HIV-1 in semen and on the sexual transmission of HIV-1 require further study.