The pharmacokinetics of intravenous (i.v.) and intramuscular (i.m.) single-dose administration of acyclovir were determined in Quaker parakeets. After i.v. injection at a dose of 20 mg/kg of acyclovir, elimination half-life was estimated at 0.65 h, volume of distribution at steady state was 627.65 ml/kg, and clearance was 11.22 ml/kg/min. The estimated pharmacokinetic values after i.m. injection at a dose of 40 mg/kg of acyclovir were an elimination half-life of 0.71 h and a bioavailability of 90.1%. The peak plasma acyclovir concentration occurred at 15 min when the drug was administered i.m. Plasma concentrations of acyclovir were undetectable 4-6 h after i.v. administration and 6-8 h after i.m. administration. Oral (capsules) and intravenous (sodium salt) formulations of acyclovir were given by gavage at 80 mg/kg. Peak concentrations with the sodium salt formulation were lower and developed more slowly than with the capsules. In studies designed to detect excessive drug accumulation or adverse side effects, acyclovir was administered i.m. at 40 mg/kg every 8 h for 7 days. Plasma concentrations were determined 15 min after (peak) and just prior to drug administration (trough). In another study acyclovir was gavaged at a dose of 80 mg/kg every 8 h for 4 days. Acyclovir plasma concentrations were determined just prior to and 2 h after drug administration. In both experiments, the birds maintained normal appetite and weight and did not exhibit excessive drug accumulation. Acyclovir plasma concentrations ranging from 2.07 +/- 1.09 micrograms/ml to 3.93 +/- 1.13 micrograms/ml were maintained for 4 days when acyclovir was administered in the feed and water (sole source of food and water).(ABSTRACT TRUNCATED AT 250 WORDS)