Lipid and protein density in each retinal layer were determined microradiographically in retinal sections of 10 rats. Lipid in retinal sections was extracted with chloroform. The fraction of dry mass remaining after lipid extraction is mainly protein. The dry mass lost in chloroform processing was calculated to obtain the lipid density, which accounts for about 30% of the total dry mass density. The total dry mass density in each retinal layer is the sum of the lipid and protein density. Lipid should be regarded as an important X-ray absorber when determining the dry mass density in the rat retina with microradiography.