Rats were treated by intramuscular injection with cortisone acetate, 25 mg./day for 5 days. Small pieces of liver obtained from treated and normal animals were squashed on a microscope slide so as to obtain many areas only a single cell in thickness. After Feulgen staining to demonstrate DNA, optical density was measured using a projection technique.
In both the normal and treated animals the nuclei were easily segregated in three ploidy classes, diploid, tetraploid, and octaploid, depending upon Feulgen intensity. In all three classes, the absorbence of nuclei from cortisone-treated animals was approximately 20 per cent lower than the normal. These data were interpreted to indicate that a change in DNA content had been induced by cortisone administration. These findings are comparable to data obtained from similar animals using chemical methods for the determination of DNA.