Alzheimer's disease (AD) is a major cause of dementia and death in the elderly, but its etiology is poorly understood. In recent years, Sparks and Schreurs (2003) have developed a rabbit model which displays 12 AD characteristics; however, the activity of brain cytochrome c oxidase (COX), which is usually low in AD, has not yet been assessed. In this study, we assessed activity of brain COX for Sparks' model. New Zealand white rabbits were maintained on the specified cholesterol-copper diet for a 10-week period following which brain mitochondria were isolated. The activity of COX within the mitochondria was assessed by polarographic assay; we also evaluated the spectral properties of the mitochondria and employed sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to investigate their protein composition. Finally, we attempted to isolate COX and studied its subunit composition by SDS-PAGE. Polarographic assay revealed that compared to the controls, 44% of the rabbits on the cholesterol-copper diet had significantly decreased brain COX activity; on average, the V (max) of the high affinity site of mitochondria from the cholesterol-copper-fed rabbits was 26% lower than that of the controls. In addition, the difference spectra of brain mitochondria obtained from 33% of the rabbits raised on the cholesterol-copper diet, showed 35-40% diminished absorbance in the 430 nm region suggesting decreased concentration or reducibility of COX or another heme protein. SDS-PAGE analysis revealed that, for the rabbits raised on the cholesterol-copper diet, a number of COX subunits (VI-VII) were loosely held and easily lost during attempts to isolate the enzyme.