The effects of mono- and dibutyl phthalate and mono- and di(2-ethylhexyl) phthalate on energy-dependent K+ uptake, respiration rates, and succinate cytochrome c reductase activities of isolated rat liver mitochondria were evaluated. The energy-coupling processes, active K+ transport and oxidative phosphorylation, were affected most by di-n-butyl phthalate and mono(2-ethylhexyl) phthalate. Mono-n-butyl phthalate had a moderate effect on energy coupling and di(2-ethylhexyl) phthalate had no apparent effect. The potency of inhibition of succinate cytochrome c reductase activity was mono(2-ethylhexyl) phthalate greater than di-n-butyl phthalate greater than mono-n-butyl phthalate = di(2-ethylhexyl) phthalate. It is concluded that phthalate esters affect mitochondrial activities by altering the permeability properties of the inner membrane and by inhibiting succinate dehydrogenase activity.