Abstract: Qinghaosu (Artemisinine) is a new antimalarial drug extracted from the Chinese madicinal herb, Qing Hao (Artemisia annua Linn.). Animal experiments and clinical observations indicate that Oinghaosu is highly effective against erythrocytic parasite with low toxicity and marked effect on falciparum malaria resistant to Chloroquine. But, in the usual clinical dosage (0.3 g im qd for 3 days), its average recrudescence rate is more than 10%. In order to reduce the recrudescence rate, the biological halflife of the drug and therapeutic effect in mice infected with Plasmodiurn berghei ANKA strain were studied.Two different methods were used for the determination of biological half-life of the drug: one was to examine the pharmacodynam.ic attenuation rate at different times after intramuscular injection, the other was to complement the SD₅₀ (half suppresive dose). The results of the two methods were similar, i.e. 10.5±2.3 h and 11 h, respectively.Two dose regimes of drug administration were compared:1. Regime A: based on t_1/2, 1/6 of the total test dose of Qinghaosu was administered intramuscularly per 12 hours with the initial dose doubled. The course of the test was 3 days.2. Regime B: referring to the dosage regime used clinically, i.e. 1/3 of the total dose was injected intramuscularly per 24 hours for 3 days. The total dose of the drug in regime B was the same as that in regime A.After treatment the animals were observed for 3 months, the results demonstrate that dosage regime A was better than dosage regime B. The curative rate in regime A was 10～40% higher than that in regime B with the same dosage. The CD₅₀ and CD₉₅ of regime A were 29.4% and 32.8%o lower respectively than those of regime B.