The pharmacokinetics of intravenously administered acyclovir were studied in 10 patients with advanced malignancies. After doses of 0.5 and 1.0 mg/kg, the slow disposition half-life values (t1/2beta) ranged from 2.2 to 3.1 h for the 1-h infusions and from 1.8 to 3.7 h for the 6-h infusions. Plasma levels, measured by radioimmunoassay, reached a maximum at the end of the 1-h infusions and approached steady state at 3 to 4 h into the 6-h infusions. Mean peak plasma concentrations obtained at 0.5 and 1.0 mg/kg administered over 1 h were 3.03 and 5.99 microM, respectively. Mean peak levels for the 6-h infusions were 1.07 microM at 0.5 mg/kg and 2.58 microM at 1.0 mg/kg. The mean urinary elimination of acyclovir was 44.7% of the administered doses. No clinical or laboratory abnormalities were noted in the 10 patients studied.