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The antimalarial activities of primaquine and its metabolites against exoerythrocytic (EE) stages of Plasmodium berghei in vitro were compared with their abilities to spontaneously generate activated oxygen. A quantitative relationship between the number of sporozoites and the number of EE merozoites produced was established. The reduction in the number of merozoites was used as an assay of drug activity. The ED50 of primaquine, 3.7–3.9 × 10-6 M, was the concentration of drug that reduced the number of merozoites to 50% of controls. Several of the primaquine metabolites were much more potent than primaquine, with ED50s as low as 2 × 10-7 M. Metabolites containing the 4-amino-1-methylbutyl side chain were most effective in vitro. Superoxide generation was measured for the various metabolites. In general, superoxide generation did not correlate with antimalarial activity. However, for the 3 metabolites with 4-amino-1-methylbutyl side chains, there was a correlation between superoxide generation and antimalarial activity.