In vitro studies on the drug response of Plasmodium falciparum were conducted as from 1998 in the district of Mae Sot in northwestern Thailand near the border to Myanmar. In vitro studies on the drug response of Plasmodium vivax started in 2001 in the same area. For P. falciparum the investigations showed a very high degree of resistance. The sensitivity to mefloquine declined significantly between 198 and 2005, whereas the sensitivity to artemisinin and to quinine increased during the same period. Sensitivity to lumefantrine and atovaquone is still in the therapeutically feasible range. The sensitivity of P. vivax to chloroquine has declined between 2000 and 2004. Mefloquine and lumefantrine have been identified as potential replacement drugs. The activity of monodesbutyl-benflumetol proved to be higher than that of structurally related lumefantrine.