Abstract: Objective To study the antimalarial activity of naphthoquine phosphate combined with artemisinine against Plasmodium knowlesi in rhesus monkey. Methods Monkeys were randomly divided into 9 groups （3/group）. The monkeys in groups A and B were treated i.g. once daily for 3 days with 6 or 10 mg/kg of naphthoquine phosphate respectively. Those in groups C and D were treated i.g. twice for the 1st day and once for the 2nd and 3rd day with 31.6 or 100 mg/kg of artemisinine respectively. In groups E, F and G, they were treated i.g. only once with the combination of naphthoquine phosphate 10 mg/kg and artemisinine 10，20 or 25 mg/kg respectively. Groups H and I served as controls which were treated i.g. only once with 10 mg/kg of naphthoquine phosphate and 30 mg/kg of artemisinine respectively. Parasitemia was examined beginning 24 h after drug administration. The observation lasted 105 days when no more parasite was found. Results At 24 h after drug administration, the parasite reduction rate in all groups was higher than 90%. The parasite clearance time for groups E, F and G was （56.0±16.0）, （53.3±4.6），and （56.0±8.0） h respectively, more rapid than that of Group H ［（69.3±4.6） h］. There were 1, 3, 3, 2, 2，and 3 monkeys in groups A, B, D, E, F，and G respectively which were cured. No monkeys were cured in groups C, H and I. Conclusion The combination of naphthoquine phosphate and artemisinine is superior to the single component and the optimum proportion in the combination is 1:2.5 in treating P. knowlesi infection in monkeys.

Key words: Naphthoquine phosphate, Artemisinine, Plasmodium knowlesi, Rhesus monkey