The present study is concerned with the analysis of Plasmodium falciparum strains from the Brazilian Amazon Region, collected at the Malaria Laboratory - SUCENT. "In vitro" sensitivity tests were performed using chloroquine (46 samples), quinine (42 samples) and mefloquine (51 samples). Results have shown "in vitro" resistance to chloroquine in 100% of the tested samples, to quinine in 2.4% and to mefloquine in 3L4%. Seven patients were treated with quinine and nine with the triple combination (mefloquine plus sulfadoxine plus pyrimethamine). No correlation was shown between the therapeutic response and the "in vitro" tests.