We investigated high-performance liquid chromatographic (HPLC) determination of 1-adamantanamine hydrochloride (1-ADA) and 2-adamantanamine hydrochloride (2-ADA) in human plasma after the derivatization with o-phthalaldehyde (OPA) and 1-thio-beta-D-glucose (TG). Extracted human plasma samples were mixed with OPA and TG at room temperature for 6 min and injected onto HPLC. Retention times of 1-ADA and 2-ADA derivatives were 12.6 and 14.1 min, respectively. The lower limits of detection of 1-ADA and 2-ADA were 0.02 and 0.008 microg/ml, and the lower limits of quantitation of 1-ADA and 2-ADA were 0.025 and 0.01 microg/ml, respectively. The coefficients of variation for intra-day and inter-day assay of 1-ADA and 2-ADA were less than 4.4 and 6.0%, respectively. L-Dopa and dopamine were not found to interfere with the peaks of 1-ADA and 2-ADA derivatives. Human plasma unbound fraction (f(p)) values of 1-ADA varied between 0.32 and 0.48, while those of 2-ADA varied between 0.38 and 0.68. These results indicate that HPLC assay of 1-ADA and 2-ADA by derivatization with OPA and TG is simple, rapid, sensitive and reproducible for determining 1-ADA and 2-ADA in human plasma.