The sensitivity of Plasmodium falciparum to chloroquine, mefloquine, quinine, quinidine, halofantrine, artemisinin, and sulfadoxine/pyrimethamine was investigated in Lambarene, Gabon in 1994. The development of in vitro susceptibility has been traced from 1983 or 1992 to 1994 for chloroquine, mefloquine, halofantrine, and quinine. Standard in vitro microtests according to World Health Organization methodology were performed. Of 33 isolates tested for susceptibility to chloroquine, 31 were resistant, one was borderline, and one isolate was sensitive (mean 50% effective concentration [EC50] = 1.38 micromol/L of blood). With mefloquine, all isolates were fully inhibited below the threshold of resistance (mean EC50 = 0.51 micrmol/L of blood). Of 32 isolates tested with quinine, six had borderline resistance (mean EC50 = 0.54 micromol/L of blood medium mixture). Susceptibility to quinidine was higher with a mean EC50 of 0.15 micromol/L of blood medium mixture. With halofantrine, 26 of 32 isolates matured at 3 nmol/L of blood medium mixture (mean EC50 = 1.64 nmol/L of blood medium mixture), indicating a steep decrease in susceptibility in comparison with 1992. For artemisinin, the mean EC50 was 97.92 nmol/L of blood medium mixture. Sulfadoxine/pyrimethamine showed five of 16 resistant isolates with a mean EC50 of 2.46 nmol/L of blood medium mixture. Whereas chloroquine resistance remained stable with a tendency to decrease, susceptibility to mefloquine and quinine was slightly decreased. A significant increase in the mean EC50 and EC90 in comparison with our previous data from Gabon was found for halofantrine.