Following intraperitoneal administration of [3H] aflatoxin B1 (AFB1) to young adult male rats, there is rapid uptake of the carcinogen by the liver, the target organ for carcinogenesis, leading to DNA covalent binding. Acid hydrolysis of this DNA shows that after 2h, the major DNA adduct is trans 8, 9-dihydro-8-(7-guanyl)-9-hydroxy AFB1 (AFB1-gua). By 24h after AFB1administration the major DNA adduct is no longer AFB1-gua but a product with the identical retention time on h.p.l.c. to 8, 9-dihydro-8-(N5-formyl-2', 5', 6' triamino-4' oxo-N5-pyrimidyl)-9-hydroxy AFB1 (AFBrtriamino-Py). 48h after carcinogen administration, only a small amount of AFB1-gua remains and the major product is AFB1-triamino-Py. The half-life of removal of AFB1gua is 22h, while AFB1-triamino-Py is much more persistent. In vitro incubation studies on DNA isolated from rats treated 2h previously with [3H] AFB1 show that at pH 7.4 AFBi-gua is the major product released from the DNA with some release of 8, 9-dihydro-8, 9-dihydroxy AFB1, (AFB1-diol). If more extensively reacted AFB1DNA is used than that obtained from in vivo administration, then the rate of AFB1-diol release is enhanced while that of AFB1gua is reduced. It would appear, therefore, that much of the release of AFB1 from DNA in vivo within the first 24h is probably not through a DNA repair process but through chemical release arising from the positively charged N7-guanine. There is considerable conversion of AFB1-gua to AFBi-triamino-Py onin vitro incubation of DNA as well as AFB1-gua and AFB1-diol release. By 24h approximately 66% of the bound AFB1 is in the form of AFB1-triamino-Py and after 48h the conversion is complete. The complex pattern of AFB1release fromDNA may have important consequences in both the induction of mutations and in tumour initiation.