EARLY effects of carcinogenic hydrocarbons on the cell cycle and on deoxyribonucleic acid (DNA) synthesis have been noted by several authors. McCarter and Quastler1,2 found that 7,12-dimethylbenz[α]anthracene (DMBA) decreases the rate of incorporation of 3H-thymidine into DNA and prolongs the DNA synthetic phase of the generation cycle of mouse epidermal cells. Evensen3 found that 3-methylcholanthrene alters the proportion of epidermal cells labelled with 3H-thymidine in the hairless mouse. Jensen et al.4 reported a reduction in uptake of 3H-thymidine in various tissues of the mouse following the administration of DMBA, including some tissues in which tumours do not usually arise after treatment with this agent. Elgjo5 reported that 3-methylcholanthrene changes the mitotic phase of the cell cycle over an eleven-week period. We now report an early effect of DMBA on the synthesis of ribonucleic acid (RNA) in mouse skin.