Three simple methods using visual titrimetric, potentiometric and spectrophotometric techniques are described for the determination of acyclovir in pure form and in pharmaceutical formulations. The methods are based on the neutralisation reaction involving the primary amino group of the drug and acetous perchloric acid in acetic acid medium. In titrimetric methods, the titration is completed with visual or potentiometric end-point detection, crystal violet being used as the indicator in visual titration. In spectrophotometry, the drug is treated with a fixed amount of perchloric acid-crystal violet mixture and absorbance of the resultant violet colour is measured at 570 nm and is related to drug concentration. Both titrimetric methods are applicable over 2-20 mg range of drug and the titration reaction follows a 1:1 stoichiometry. In spectrophotometry, Beer's law is obeyed over the concentration range 5-55 mug mL(-1) with an apparent molar absorptivity and Sandell sensitivity of 1.78 x 10(4) L mol(-1) cm(-1) and 12.68 ng cm(-2), respectively. The limits of detection and quantification are calculated to be 1.696 and 5.654 mug mL(-1), respectively. The methods were successfully applied to the determination of acyclovir in tablets. The validity of the methods was further ascertained by parallel determination by a reference method and by recovery studies via standard-addition technique.