The contribution of different steps to the control of oxidative phosphorylation in isolated rat liver mitochondria was investigated by a combination of experiments and computer simulations. The parameters of the mathematical model of phosphorylating mitochondria were derived from experimental data. The model correctly described the competition between ATP utilization inside and outside mitochondria for the ATP generated in mitochondria. On the basis of the good agreement between experiments and simulations, the contribution of different steps to the control of respiration was estimated by computing their control strengths, i.e., the influence of their activities on the rate of respiration. The rate-controlling influences vary depending on the load of oxidative phosphorylation. The predominant steps are: in the fully active state (State 3)--the hydrogen supply to the respiratory chain; in the resting state (State 4)--the proton leak of the mitochondrial inner membrane; in states of non-maximum ATP export--the adenine nucleotide translocator. Titrations of respiration with phenylsuccinate, antimycin, oligomycin and carboxyatractyloside completely support these conclusions.