It is known that tumours of several endocrine glands can be induced by a combination of a physiological stress and radiation. It was decided to assess the effect of radiation and of changes in dietary calcium on the development of thyroid tumours in the rat. Three hundred rats were given either 0, 5 or 10 μCi of 131I in their first day of life. Each of these groups was subdivided after weaning, and maintained on a diet that was either high, normal or low in calcium. The animals were killed at intervals up to 27 months of age, and the numbers of thyroid tumours recorded. Follicular tumours were first noted at 9 months of age, and their frequency increased steadily with age. The effect of radiation was highly significant, only one tumour occurred in a non-irradiated animal. There was a small increase in frequency in follicular tumours in the high calcium diet group as compared to the low calcium diet group. C cell tumours were first noted at 9 months of age, and their incidence again increased with age. Significantly more tumours occurred in the radiated than in the non-irradiated animals. No significant variation occurred in relation to dietary calcium. It is concluded that an increase in dietary calcium, known to be mildly goitrogenic, may also be important in the carcinogenesis of follicular but not C cell tumours, and that radiation, known to be carcinogenic for thyroid follicular cells is also carcinogenic for C cells.