Curcumin, a constituent of the traditional Indian spice and medicine turmeric, was evaluated for its capacity to inhibit the mammary tumor initiating activity of 7,12-dimethylbenz[a]anthracene (DMBA) and the in vivo formation of mammary DMBA-DNA adducts in the female rat. Administration (i.p.) of curcumin at 100 mg/kg and 200 mg/kg doses was associated with a significant decrease in the number of palpable mammary tumors and mammary adenocarcinomas. The in vivo formation of mammary DMBA-DNA adducts also was depressed for animals administered curcumin doses from 50 mg/kg to 200 mg/kg. There was, however, no significant enhancement of liver glutathione-S-transferase activity following curcumin administration. Therefore, curcumin when administered i.p. can act as an effective chemopreventative agent towards DMBA-induced rat mammary tumorigenesis and mammary adduct formation.