Rapid, simple, and selective methods for determining amantadine HCI and acyclovir antiviral drugs in pharmaceutical and plasma samples were developed using 1H-NMR spectroscopy with dimethyl sulfoxide (DMSO-d6) as the solvent. Integrations of the 1H-NMR signals at 2.07 and 7.82 ppm were used, respectively, for quantifying the two drugs, with the malonic acid signal at 3.24 ppm as the internal reference signal. Average recoveries of 98.24-101.00 +/- 4.82% and 97.7-100.38 +/- 3.36% were obtained for amantadine HCI and acyclovir in pharmaceutical samples, respectively. Average recoveries of 97.36-103.68 +/- 2.99 and 93.81-99.80 +/- 2.93 were obtained, respectively, for both drugs in plasma samples. The statistical Student's t-test gave t-values < or = 1.41 for analyzed pharmaceutical samples and t-values < or = 0.29 for analyzed plasma samples. These values indicated insignificant difference between the real and measured contents at the 95% confidence level. Application of the statistical F-test for the analytical results of amantadine HCI gave F-values < or = 6.44 and 2.80 in pharmaceutical and plasma samples, respectively. F-values < or = 6.82 and 3.86 were obtained for acyclovir in pharmaceutical and plasma, respectively. These values indicated insignificant differences in precisions between the developed NMR methods and arbitrarily chosen HPLC methods reported for determining both drugs in pharmaceutical and plasma samples.