Although the function of peroxisomes is poorly understood, they appear to be especially abundant in cells involved in cholesterol metabolism and synthesis of steroids - liver, adrenal, ovary, and interstitium of the testis. A relationship to cholesterol metabolism is suggested by the observation that the number of hepatic peroxisomes is dramatically increased by administration of several unrelated hypocholesterolemic drugs. The upper micrograph from normal rat liver, stained with the diaminobenzidene reaction illustrates the normal size of number of peroxisomes. The lower micrograph shows the remarkable increase after the administration of acetic acid.