Respiration of early-exponential phase cultures of the ciliate protozoon Tetrahymena pyriformis is inhibited in two stages with increasing concentrations of cyanide. Up to 40 to 50 % inhibition occurs at low concentrations (< 15 PM). Maximal inhibition was obtained with 300pwcyanide; at this concentration 20 % of the respiration was still unaffected.Competitive inhibition of respiration by CO occurs (Ki = 4-25 p~) ; azide inhibition of oxygen consumption is uncompetitive (K, = 4 to 9 mM). A salicylhydroxamic acid-sensitive oxidase is present which is not inhibited by cyanide, azide, CO or H,S. Three other pathways of terminal oxidation are present which are insensitive to azide, CO and salicylhydroxamic acid. One of these is cyanide-and sulphide-sensitive, a second is cyanide-insensitive but sulphide-sensitive, and a third is cyanide-sensitive but sulphide-insensitive ; these pathways may not possess unique terminal oxidases. No oxidase with a very low affinity for oxygen was detected, but the overall affinity for oxygen of T. pyriformis in the absence of inhibitors (K, = 2.9 PM) is lower than that of some other protozoa.