For an accurate histological evaluation of cochlear blood flow, it is essential to fix the cochlear vessels while maintaining their physiological state. In the present study, we administered 10% CO2 to guinea pigs and then used phase-contrast microscopy to determine how two different methods of fixation influenced the cochlear vasculature. The first method of fixation employed perilymphatic perfusion in vivo, while the second one involved fixation after decapitation. Decapitation caused significant changes in the vessels of the stria vascularis, including constriction and sludging. In contrast, no sludging occurred in the perilymphatic perfusion method and erythrocyte morphology was preserved. However, dilatation of the strial blood vessels occurred after the inhalation of 10% CO2 even in the decapitation method. The results demonstrate that particular attention must be paid to the fixation method used, especially when evaluating the blood flow of the stria vascularis.