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Summary.-Adult mice of strains C3Hf and XVII/G received 2,4,5-T by continuous oral administration (80 ppm in the diet). The 2,4,5-T preparation contained less than 005 ppm of dioxins. In 2,4,5-T-treated C3Hf mice a significant increase in the incidence of neoplastic lesions was found. No significant difference was found in the XVII/G strain between the treated and control mice. Rare forms of tumours, which were not observed in the controls, were present in the 2,4,5-T-treated C3Hf mice.

Thymectomized AkR mice were treated with ¹³¹I, thyroxin and thiourea. No direct relationship were found between thyroid function disturbances and the frequency of intracranial bone tumours. Mice with induced thyrotoxicosis (0.1 mg of thyroxin/day/mouse) and radiothyroidectomited mice (30 µC_i of ¹³¹I) presented the same frequency of osteomas as the controls. Administration of thiourea (0.5 g/kg mouse/day) increased very significantly the frequency of the tumours. The possibility that this effect of thiourea administration arises, not from its inhibitory effect towards the synthesis of thyroxin, but from hypothalamic lesions is discussed.

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