Alkaline solutions (0.1-0.5 N NaOH) at elevated temperatures can be used to extract DNA from small pieces of tissue. RNA is destroyed by the treatment. In tissues which were previously exposed to tritiated thymidine, aliquots from the extracting solution can be used directly for the determination of DNA by ethidium bromide fluorescence and radioactivity in DNA and the nucleotide pool. [Guinea pig and human skin were studied.].