The mitochondrial respiratory parameters were measured in situ, i.e. in saponin-skinned rabbit cardiac fibers and in fibers treated with saponin + collagenase. It was found that the decrease of maximal ADP-stimulated respiration rate of saponin-skinned fibers with pyruvate + malate under the conditions of total ischemia (0.5-1.5 h) is less pronounced as compared to isolated mitochondria. Maximal succinate oxidation rate (+ADP), however, was not different from control (1 h ischemia) but it exceeded the control level when measured in the medium supplemented with cytochrome c. It was also demonstrated that treatment of fibers with collagenase alone or in combination with saponin significantly (almost 2 fold) enhanced the maximal ADP-stimulated respiration rate if compared with saponin-skinned fibers. The data obtained suggest that mitochondrial respiration in saponin-skinned rabbit cardiac fibers is not completely revealed, most probably, due to insufficient permeabilization of sarcolemma by saponin and, thus, inadequate accessibility of mitochondria to exogenous substrates, ADP in particular. These parameters can be improved by pre-treatment of fibers with collagenase.