The electrochemical properties of valacyclovir, an antiviral drug, were investigated in pH range 1.8-12.0 by cyclic, differential pulse and square-wave voltammetry. The drug was irreversibly oxidized at a glassy carbon electrode in one or two oxidation steps, which are pH-dependent. For analytical purposes, a very resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 10.0 using differential pulse and square-wave modes. Limits of detection were 1.04 x 10(-7) and 4.60 x 10(-8) M for differential pulse and square-wave voltammetry, respectively. The applicability to direct assays of tablets, spiked human serum and simulated gastric fluid, was described. (c) 2005 Elsevier B.V. All rights reserved.