Postmortem changes in mitochondrial respiratory enzymes (Complex I-IV and NAD(+)-linked dehydrogenases in the TCA cycle) were studied in mouse brains and human frontal lobes. In mouse brains, activities of the enzymes studied were generally stable for as long as 12 h after cervical dislocation, except for the alpha-ketoglutarate dehydrogenase complex and NADP(+)-linked isocitrate dehydrogenase. In human frontal cortices, only NADH-ubiquinone reductase (Complex I) activity showed significant negative correlation with the duration between the patient's death and the freezing of the brain. No correlations between the activities of the enzymes studied and the age of the patients were noted. As most of our patients were 50 years of age or above, absence of the correlation cannot be extended to younger patients. From our observation, it was felt that analyses of these mitochondrial enzymes in human autopsy brains would give meaningful data. Preliminary observation in Parkinson's disease revealed a small but a significant decrease in the activity of Complex III in the striatum as compared with the control. Although, significance of our observation is not yet known, further studies on this line appear to be important to elucidate pathogenesis of Parkinson's disease.