Malagashanine (MG) is the parent compound of the new N(b)-C(21) seco-curan alkaloids isolated hitherto from Madagascan Strychnos. On account of its promising chemosensitizing activity against chloroquine (CQ)-resistant Plasmodium falciparum (Pf) strains, we investigated its mechanism of action. One prominent result was the finding that MG significantly increased the accumulation of [3H]-chloroquine in chloroquine resistant (CQR) K1 and FCM29 Pf strains at mid and old trophozoite stages. This effect was concentration-dependent and not observed in chloroquine sensitive (CQS) strains. Comparative monitoring of the release of [3H]-CQ from pre-loaded CQR and CQS Pf strains revealed strong concentration-dependent inhibition of CQ release by MG from the pre-loaded CQR FCM29 strain. We also found that MG substantially inhibited the loss of pre-accumulated CQ in the resistant K1 strain after a washing procedure at 4 degrees C. On the other hand, we observed that the addition of glucose at the old trophozoite stage induced a rapid increase in CQ accumulation in the CQS 3D7 strain cultured in glucose-free medium, but not in the CQR FCM29 strain. Interestingly, MG considerably increased (>100%) CQ accumulation in the FCM29 strain in a glucose-free medium while the addition of glucose further significantly increased this accumulation. Our study therefore clearly demonstrates that MG prevents CQ efflux from, and stimulates CQ influx into, drug-resistant Pf. Overall, MG appears to be a useful lead for the design and synthesis of more powerful and effective resistance modulators.