The toxic effects of artesunate and dihydroartemisinin on the growth metabolism of Tetrahymena thermophila BF5 were studied by microcalorimetry. The results showed that: (1) low concentrations of artesunate (<or=1 mg L(-1)) and dihydroartemisinin (<or= 2 mg L(-1)) promoted the growth metabolism of T. thermophila BF5, whereas high concentrations of artesunate (1-60 mg L(-1)) and dihydroartemisinin (2-60 mg L(-1)) inhibited its growth; (2) the half inhibition concentrations IC(50) of artesunate and dihydroartemisinin were 17.5817 and 9.5089 mg L(-1), respectively. It was concluded that the inhibition of dihydroartemisinin was stronger than that of artesunate.