The use of the antimalarial agent amodiaquine has been curtailed due to drug-induced idiosyncratic reactions. These have been attributed to the formation of a protein-reactive quinoneimine species via oxidation of the 4-aminophenol group. Therefore, the effects of chemical modifications on the disposition of amodiaquine in relation to its metabolism, distribution, and pharmacological activity have been investigated. The inclusion of a group at the C-5' position of amodiaquine reduced or eliminated bioactivation, as determined by glutathione conjugate formation in vivo. This can be seen in two series of C-5'-substituted compounds: the bis-Mannich antimalarial agents, including cycloquine and pyronaridine, and mono-Mannich antimalarial agents containing a 5'-chlorophenyl group (tebuquine and 5'-ClPAQ). Chemical substitution at the C-5' position also resulted in compounds which underwent slower elimination (<5% of the dose excreted into bile and urine, compared with 50% for amodiaquine) and increased levels of accumulation in tissue (10% of the dose in the liver at 48 h compared with 1% with amodiaquine). This may be due to an increase in either the lipophilicity or the basicity of the analogs and may reflect the lack of metabolic clearance for these compounds. The alteration in the disposition following the introduction of the C-5' substituent resulted in an increased duration of antimalarial activity in the mouse compared with that for amodiaquine. While this is desirable in the treatment of malaria, repeated administration for prophylaxis may induce toxicity through accumulation. Therefore, by simple chemical modification it is possible to block the bioactivation of amodiaquine while maintaining and in some cases extending the duration of antimalarial activity.