Objective: To investigate the association between amplification of the two regulatory genes controlling glutathione (GSH) levels, glutathione reductase (P. fGR) and glutathione S-transferase (P. fGST) genes and sensitivity of Plasmodium falciparum (P. falciparum) isolates collected from different malaria endemic areas of Thailand to standard antimalarial drugs. Methods: A total of 70 P. falciparum isolates were collected from endemic areas of multi-drug resistance (Tak, Chantaburi and Ranong Provinces) during the year 2008-2009. The in vitro assessment of antimalarial activity of P. falciparum clones (K1- and Dd 2 chloroquine resistant and 3D7-chloroquine sensitive) and isolates to chloroquine, quinine, mefloquine and arteusnate was performed based on SYBR Green modified assay. Results: 68 (97.14%), 11 (15.71%) and 28 (40%) isolates respectively were classified as chloroquine-, quinine- and mefloquine-resistant isolates. With this limited number of P. falciparum isolates included in the analysis, no significant association between amplification of P. fGST gene and sensitivity of the parasite to chloroquine, quinine, mefloquine and quinine was found. Based on PCR analysis, Dd2, K1 and 3D7 clones all contained only one copy of the P. fGST gene. All isolates (70) also carried only one copy number of P. fGST gene. There appears to be an association between amplification of P. fGR gene and chloroquine resistance. The 3D7 and Dd2 clones were found to carry only one P. fGR gene copy, whereas the K1 clone carried two gene copies. Conclusions: Chloroquine resistance is likely to be a consequence of multi-factors and enzymes in the GSH system may be partly involved. Larger number of parasite isolates are required to increase power of the hypothesis testing in order to confirm the involvement of both genes as well as other genes implicated in glutathione metabolism in conferring chloroquine resistance. © 2010 Hainan Medical College.