The rate of dark O(2) uptake of Elodea canadensis leaves was titrated with either cyanide or sulfide in the presence and in the absence of 5 millimolar salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase. The inhibition of O(2) uptake by SHAM alone was very small (3-6%), suggesting that actual respiration mainly occurred through the cytochrome pathway. O(2) uptake was slightly stimulated by cyanide at concentrations of 50 micromolar or higher, but in the presence of SHAM respiration was strongly suppressed. The effects of sulfide on O(2) uptake were similar to those of cyanide, except that the percent stimulation of O(2) uptake by sulfide alone was somewhat higher than that of cyanide. However, the estimates of the capacity of the alternative pathway were similar with both inhibitors. Another difference is that maximal inhibition of respiration in the presence of SHAM was observed with lower concentrations of sulfide (50 micromolar) than cyanide (250 micromolar). The results suggest that sulfide can be used as a suitable inhibitor of cytochrome c oxidase in studies with intact plant tissues, and that sulfide does not apparently inhibit the alternative oxidase.