Doxycycline is reported to impair second-generation parasite schizogony. The effects of doxycycline alone and combined with dihydroartemisinin were investigated in a murine malaria model. Doxycycline lowered the rate of parasite growth within 2 days, with maximum effect in 6 days. Addition of dihydroartemisinin led to an additive antimalarial effect.Tetracyclines are relatively potent antimalarial drugs with slow/delayed onsets of action (8,10,17,19) and low parasite reduction ratios (20). Hence, it is recommended that tetracyclines be used in combination with fast-acting antimalarials, such as quinoline or artemisinin drugs, for extended durations of at least 7 days (14,17,(20)(21)(22). Doxycycline (DOX) is widely used in malaria prophylaxis and is acceptable for long-term therapy, except in children and pregnant women (22). Furthermore, DOX combined with quinine is indicated for standby emergency treatment of falciparum malaria (22), and DOX has demonstrated efficacy in combination with artesunate, albeit with a 20% recrudescence rate that was attributed to an inadequate dosage of one or both drugs (13).Despite the limited clinical role, in vitro and animal investigations have shown that DOX and other tetracyclines are effective antimalarial drugs against drug-resistant parasite strains (4, 10, 12). However, combination studies of DOX and artemisinin drugs have produced conflicting reports of additive (9, 23) and synergistic (7, 19) effects, thus highlighting the difficulty in predicting clinical outcomes from in vitro experiments and animal studies.While differentiation between synergistic and additive effects may be problematic, interpretation of pharmacodynamic data should be more feasible with the recent elucidation of the mechanism of action of tetracyclines (8). Dahl et al. (8) showed that DOX caused a parasite organelle, the apicoplast, to become dysfunctional in progeny parasites. Late-stage parasites were most susceptible to DOX, and the drug effect was evident in mature parasites from the second generation, which did not rupture to release viable merozoites.With a background of conflicting in vitro data, clinical observations of delayed effect, and the recently reported mechanism of action, we investigated the effect of DOX alone and in combination with dihydroartemisinin (DHA) in a murine malaria model. The method used in our study was adapted from the Rane test (15) in order to evaluate the drug effect on parasite elimination in an established infection and to obtain pharmacodynamic data from a treatment model.