Activated ras oncogenes have been detected in a number of rodent tumour model systems (Balmain et al., 1986; Barbacid 1987), in which chemical carcinogens have been the most widely used inducing agents. We have investigated the involvement of activated oncogenes in a single well-characterised tumour model, rat thyroid neoplasia, produced not only by chemical carcinogen (nitrosomethylurea, NMU) and by ionizing radiation but also by elevated trophic stimulation alone. We have indentified a striking specificity in patterns of ras oncogene activation, with exclusively Ha-ras activated in 87% of NMU-induced tumours and exclusively Ki-ras activated in 60% of radiation-induced tumours.