Owing to the low plasma concentrations of acyclovir obtained during pharmacokinetic studies after low-dosage oral administration, a sensitive, automated HPLC method had to be developed for determining acyclovir in a large number of plasma samples. Extraction and injection of the samples were done automatically by a Gilson ASPEC system using tC18, 100-mg Sep-Pak Vac extraction columns. The extracts were chromatographed on a Nova-Pak C18 column with sodium octanesulphonate and methanol in the mobile phase. The analyte was detected at 250 nm. The calibration graphs were linear up to at least 1200 ng/ml and the limit of quantification was 10 ng/ml.