The effect of 2,3,5-trichloro-4-pyridinol (pyriclor), as well as carbonyl cyanide m-chlorophenyl hydrazone, 2,4-dinitrophenol, and oligomycin, on oxygen uptake and oxidative phosphorylation was investigated with mitochondria isolated from hypocotyls of etiolated soybeans (Glycine max Merrill, var. Lee). Pyriclor inhibited both coupled and uncoupled reactions. This suggests that pyriclor either acts on a nonphosphorylating intermediate close to the electron carrier chain or with a component of the electron carrier chain, or both.