The sensitivity of Plasmodium falciparum to chloroquine, mefloquine, quinine, halofantrine, and sulfadoxine/pyrimethamine has been investigated at Lambaréné, in the Gabonese rain forest, between April and September 1992. WHO standard micro in vitro tests were performed. Of 43 isolates tested for response to chloroquine all were resistant to the drug with mean EC 50 and EC 90 values of 1.86 and 4.18 mumol/l blood, respectively, indicating the highest degree of resistance ever reported from Central Africa. With sulfadoxine/pyrimethamine 19 out of 27 isolates showed 90% inhibition of schizont maturation at a pyrimethamine concentration of at least 75 nmol/l blood medium mixture, indicating 30% of resistance to sulfadoxine/pyrimethamine. In contrast all isolates tested were fully inhibited by mefloquine at 3.2 mumol/l blood (40 isolates), quinine at 5.12 mumol/l blood medium mixture (41 isolates) and halofantrine at 3 nmol/l blood medium mixture (40 isolates) indicating full sensitivity to these drugs. A significant positive correlation was found between responses to quinine and mefloquine. The response to halofantrine was positively correlated with the responses to quinine and mefloquine, in the case of chloroquine and halofantrine an inverse relationship was observed. Compared with previous data from Gabon, the findings suggest a substantial increase of chloroquine resistance, in contrast to reports from neighbouring countries, which show stabilising or even declining chloroquine resistance patterns.