The 8-aminoquinolines were the first group of compounds to be synthesized specifically for their antimalarial activity. A large-scale research programme in the United States of America in the 1940s produced three new antimalarial drugs-pentaquine, isopentaquine, and primaquine -of which primaquine was the most effective. This article reviews knowledge on the activity of these 8-aminoquinolines against all stages of the malaria parasite and suggests their possible modes of action. The toxic effects and possible causal mechanisms are also outlined.