Tamoxifen is an anti-oestrogen widely used in the adjuvant therapy of breast cancer and is also used as a prophylactic to prevent the disease in high-risk women. An increased risk of endometrial cancer has been observed in both settings. In rats, tamoxifen potently induces liver carcinomas and also induces uterine tumours when given neonatally. It forms DNA adducts in rat liver via the formation of alpha-hydroxytamoxifen, the ultimately reactive form being generated by sulfotransferase. In order to investigate the formation of tamoxifen-derived DNA adducts in other rat tissues, female Fischer F344 or Sprague-Dawley rats were treated with tamoxifen or alpha-hydroxytamoxifen by gavage or by intraperitoneal injection, daily for 1, 4 or 7 days, and DNA adducts were detected by (32)P-postlabelling analysis. Tamoxifen formed DNA adducts in the liver but not in other tissues (uterus, stomach, kidney, spleen and colon). alpha-Hydroxytamoxifen also formed adducts at high levels in liver, but with the exception of single animals (1/8) in which a low level of adducts was detected in the stomach in one case, and in the kidney in the other; it also did not give rise to adducts in other tissues. The results suggest that tamoxifen is a genotoxic carcinogen in rat liver, but a non-genotoxic carcinogen in rat uterus, making it, uniquely, a carcinogen with more than one mechanism of action. Mutagenicity experiments conducted in Salmonella typhimurium strains expressing bacterial or human N,O-acetyltransferase did not provide evidence that either alpha-hydroxytamoxifen or alpha-hydroxy-N-desmethyltamoxifen undergoes metabolic activation by acetylation. The confinement of ST2A2, the isozyme of hydroxysteroid sulfotransferase that can activate the compounds, mainly to rat liver is the possible reason for the formation of ducts in the liver but not in other organs of the rat.