Oxygen uptake in cysts and trophozoites of the parasitic protozoan Giardia lamblia was examined. Both showed oxygen uptake activity, but that of cysts was only 10% to 20% that of trophozoites. Oxygen dependence of oxygen uptake in cysts and trophozoites showed oxygen maxima above which oxygen uptake decreased. The oxygen concentration at which the oxygen uptake rate was greatest was higher for trophozoites than for cysts. The effect of various inhibitors on cyst and trophozoite oxygen uptake suggested that flavoproteins and quinones play some role in oxygen uptake. The substrate specificities and the effect of inhibitors on G. lamblia trophozoites were similar to those observed for G. muris. Metronidazole, the drug most commonly used in treatment of giardiasis, inhibited oxygen uptake and motility in trophozoites; however, it had no obvious effect on either oxygen uptake or excystation in cysts. Menadione, a redox cycling naphthaquinone, first stimulated, then completely inhibited, oxygen uptake in cysts and trophozoites; a complete loss of cyst viability and trophozoite motility was also observed. The effect of menadione on G. lamblia may indicate that redox cycling compounds have potential as chemotherapeutic agents for the treatment of giardiasis.