The ubiquitous bracken fern (genus Pteridium) is the only higher plant known to cause cancer naturally in animals. In addition to the well-recognised syndromes of thiamine deficiency, acute haemorrhage associated with myeloid aplasia and blindness due to retinal degeneration, it causes neoplasia of the urinary bladder and in some circumstances, of the upper gut. In addition, it has been shown to cause neoplasia in a wide range of tissues in many experimental species. Ptaquiloside (PT) has been identified as the cancer causing principle and is believed to be converted to an electrophilic dienone under alkaline conditions to its activated form (APT). Our previous study has shown that this activated PT alkylates DNA at N3-adenine and N7-guanine in a sequence selective fashion. We provided further evidence using calves fed on bracken diet for the activation of H-ras oncogene via initial alkylation of adenine residue in codon 61 followed by depurination at that site. In this work Sprague-Dawley rats were exposed to PT and its activated form APT intravenously for ten weeks. This route of administration requires a tenth of the concentration needed in intragastric studies. During the dosing period blood samples were collected for haematology. The animals were put down after keeping them alive for a further 30 weeks and tissues from various organs were collected and analysed for DNA damage using 32P- postlabelling and mutation studies. Histopathology examination clearly showed PT-associated mammary gland adenocarcinomas. Mutations in codon 59 of H-ras were found in preneoplastic cells but not in the tumour itself. In addition the haematology results identified high monocytosis and corresponding elevation in TNF α levels in all the treated animals. Based on our findings we propose a multistage cancer model for PT carcinogenesis.