Poliovirus infection of COS-1 and T47D cells caused a rapid decrease in total cell respiration, and this was attributed to an inhibition of mitochondrial respiration. The stimulation of mitochondrial respiration by pyruvate plus malate or succinate was impaired in saponin-permeabilised cells. However, this inhibition could be overcome by the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine and ascorbate. The activity of succinate dehydrogenase was impaired in parallel with the inhibition of mitochondrial respiration during poliovirus infection. This shows that mitochondrial function is profoundly altered during poliovirus infection and that this occurs primarily through inhibition of electron flow at complex II of the mitochondrial respiratory chain,