To understand the evolution of drug-resistant forms of malaria in time and in space, we carried out an analysis of the results of a series of passive and active surveys conducted in Burkina Faso between 1982 and 1991. A total of 607 tests for resistance to chloroquine and mefloquine were carried out in vitro and 3,679 tests for resistance to chloroquine, quinine, and sulfadoxine-pyrimethamine were performed in vivo. The surveys principally involved the two main cities of Burkina Faso, Ouagadougou and Bobo-Dioulasso. However, another 10 locations representing the three different zones of malaria transmission were also studied. The first cases of Plasmodium falciparum resistant to chloroquine in vitro were reported in 1983, but it was only in 1988 that in vivo resistance appeared. The first cases of in vitro resistance to mefloquine were noted in 1987 while chloroquine sensitivity at a high rate (15.8%), which decreased during the following years. The prevalence of resistance to chloroquine increased in parallel to this decrease in sensitivity to an overall peak of 41% in vitro and 16% in vivo in 1990. These rates then decreased to 3% and 6%, respectively, in 1991. This pattern of decreasing resistance was broadly similar in all sites except for the town of Bobo-Dioulasso, where the level of resistance remained stable at approximately 14% from 1988 to 1991. Only two cases of resistance in vivo to sulfadoxine-pyrimethamine were noted.(ABSTRACT TRUNCATED AT 250 WORDS)