The effects of artesunate(Art) and four classical antimalarials on the mitochondrial ferrochelatase(FC) activity in isolated rat liver were studied with pyridine hemochromogen method. It was shown that Art markedly inhibited the FC activity whether ferrous or ferric ion was used as substrate, but the heme biosynthesis from ferric ion was more susceptible to Art. The IC₅₀ of Art on FC was 165 μmol·L^-1 for Fe²⁺ and 30.2 μmol·L^-1 for Fe³⁺ substrate, respectively. It meant that Art not only inhibited the FC activity directly, but also interfered with Fe³⁺ reduction and/or utilization by mitochondria. So that Art could produce stronger inhibitory effect on heme biosynthesis, and the effect might be one of the toxic mechanisms of Art on erythropoiesis. The inhibition of FC activity was not found with 1 mmol·L^-1 chlo- roquine, quinine, cyclochloroguanide or primaquine treatment.