Abstract: In order to elucidate the mode of action and toxic effect of gossypol, its effects on the incorporation of H³-thymidine (H³-TdR) into DNA of the regenerative liver, testis and small intestine of adult male rats, the liver, small intestine and thymus of preadult male rats and the liver and small intestine of adult male mice have been studied. An antimetabolite-5Fu was studied in the experiments for comparison. The results showed that 5Fu inhibited significantly the incorporation of H³-TdR into DNA of the thymus, small intestine and liver of "regenerative liver" but there was no significant change in the tissues of the gossypol-treated animals, which seemed to indicate a lack of effect of gossypol on the synthesis of DNA. However, when the dosage of gossypol was increased so that atrophy of the testis appeared, the incorporation of H³-TdR and the specific activity of DNA in the testis was higher in the experimental than in the untreated control animals. A discussion of these results was given.