During the first six months of formalin fixation LHA values decrease about 20% in both grey and white matter. RHA does not decrease in the white matter but increases in the grey matter. Determination of LHA and RHA after six months of fixation is accurate enough for quantitative biological and pathological studies. During development there is an increase and subsequent fall in RHA in grey and in white matter. Maximum values are reached earlier in white than in grey matter. LHA and RHA were normal in grey and white matter in demyelinating conditions of non-metabolic origin. Increased LHA values were observed in white matter of metachromatic leucodystrophy, Tay-Sachs disease, and some other leucodystrophies. RHA increase invariably was co-occurring with LHA increase in the white matter but not so in the grey matter.