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A localized transplantable Leydig cell tumour of the Fischer rat regularly produces lethal hypercalcaemia in thyroparathyroidectomized and intact hosts. The parabiosis model was used to explore and compare the hypercalcaemia producing action of this tumour, parathyroid hormone, and vitamin D. Administration of exogenous parathyroid hormone (10 and 20 units) to one thyroparathyroidectomized parabiont rat increased the serum calcium in both parabionts within 24 hours. Endogenous parathyroid hormone, studied by producing parabiosis between thyroparathyroidectomized rats and intact rats, raised the serum calcium of the thyroparathyroidectomized parabiont within 7 days in parabiosis, suggesting that endogenous parathyroid hormone is effective across the parabiotic union. Vitamin D3 (100 000 and 150 000 units) given to one parabiont of a thyroparathyroidectomized pair produced hypercalcaemia only in the injected parabionts. When the hypercalcaemia producing tumour was grown in thyroparathyroidectomized pairs for 3\p=n-\6 weeks, severe hypercalcaemia and death occurred only in the tumour bearing parabiont. The serum calcium of the non-tumour parabiont did rise over a period of 3\p=n-\6weeks and was compatible with an approach to calcium

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