The current status of pharmacokinetic and toxicologic information on acyclovir is reviewed from a clinical pharmacologist's perspective. Acyclovir pharmacokinetics is accurately described by a two-compartment open model. The volume of distribution at steady state is about two-thirds of the body weight. The half-life of its beta phase of elimination is about three hours with normal renal function and increases to about 18 hours with anuria. Hemodialysis removes about 60 percent of the acyclovir in the body. The pharmacokinetics is independent of dose at least up to 15 mg/kg. Acyclovir is minimally (15 percent) protein-bound an only a small percentage (15 percent) of an intravenous dose is metabolized in persons with normal renal function. Acyclovir is eliminated primarily by glomerular filtration with a small addition from tubular secretion. The toxicity of acyclovir seems to be acceptably low. Local irritation with extravasation exists. Transient glomerular dysfunction is occasionally seen after bolus administration. Other side effects remain to be clearly established.