The influence of formaldehyde on rat brain lipid was investigated by comparing the analysis of lipids from fresh rat brain with the analysis from rat brains fixed for 110 days in a formaldehyde solution. A quantitative analysis of the lipids using column and paper chromatography is described. Sphingomyclin, cerebrosides and sulphatides were not attacked. Only a slight diminution of phosphatidylserine was indicated. Phosphoinositides remain fairly constant. Of lecithin about 25% was converted to lyso-lecithin. The phosphatidylethanolamine and lyso-phosphatidylethanolamine fractions suffered a marked diminution and the reaction with formaldehyde was noticeable in a change of chromatographic behaviour. The fraction containing cholesterol, cholesterol esters, glycerides, and fatty acids increased, the increase being correlated with the decrease of other fractions by the formation of lyso compounds. After storing tissues for months in formaldehyde solutions a reliable analysis of the lipid content is still possible.