Acyclovir pharmacokinetics has been extensively investigated during the various phases of clinical development. Most of the administered drug is eliminated from the body unchanged, via the kidneys by glomerular filtration and tubular secretion. After intravenous dosing of patients with normal renal function, 8 to 14% of the dose is recovered in the urine as the metabolite 9-carboxymethoxymethylguanine. Adequate distribution of acyclovir has been demonstrated in the cerebrospinal fluid, vesicular fluid, vaginal secretions and tissues. The low plasma protein binding of acyclovir precludes drug interactions involving binding displacement. When intravenous doses in the range of 2.5 to 15 mg/kg were given every 8 h to adult patients, dose-independent kinetics was observed. Continuous infusions of acyclovir over an equivalent daily dose range have achieved predictable plasma levels. Acyclovir half-life (T1/2 beta) and total body clearance (Cltot) are influenced significantly by renal function, and dosage adjustments should be made for patients with impaired renal function. For patients with normal renal function (creatinine clearance (Clcr) greater than 80 ml/min/1.73m2) mean T1/2 beta and Cltot were 2.5 h and 327 ml/min/1.73 m2, respectively. In children 1-year-old or older, Cltot normalized by body surface area was essentially the same as in adults with normal renal function, whereas Cltot for neonates was approximately one-third the adult value. In the adult population, age-related decreases in acyclovir Cltot reflect age-related changes in renal function; therefore dosage adjustments based on Clcr will compensate for age effects on acyclovir pharmacokinetics. After oral administration, the bioavailability of acyclovir was approximately 20%.(ABSTRACT TRUNCATED AT 250 WORDS)