Over the course of many years our laboratory has been engaged in the study of physiological functions of mitochondria ex vivo. We showed that the unavoidable destruction of mitochondrial-reticular network during traditional isolation of the mitochondria diminishes the observable ex vivo changes of mitochondrial processes in vivo. Comparing preparations obtained from quiescent and stressed rats, we found that the great difference in size of assemblies of mitochondria preserved in homogenate disappears when it is diluted for the measurement of respiration. This also leads to a decrease in the difference between respiration of mitochondria from quiescent and stressed animals. We developed a new method that provides ex vivo stable preservation of the in vivo network using a cytochemical procedure on glass-adhered lymphocytes in blood smear. We radically changed the incubation medium for the measurement of dehydrogenase activity that excludes an artefact of succinate dehydrogenase hyperactivation ex vivo by non-physiological components of the traditionally used solution. Our method made it possible to observe ex vivo two- to eightfold increase in succinate dehydrogenase activity by adrenaline in vivo, while the activity of alpha-ketoglutarate dehydrogenase changed reciprocally. The data obtained show that the structure changes of the network play an important role in physiological regulation of mitochondrial functions. Thus, it may be possible to correct mitochondrial dysfunctions in the organism by substances supporting the stability of mitochondrial network. The developed method is non-invasive, informative and, therefore, is convenient for clinical investigations, particularly of mitochondrial diseases.