In eight patients with classical Huntington's chorea hypothalamic function was assessed by the insulin tolerance test, the thyrotrophin releasing hormone test, the gonadotrophin releasing hormone test and water deprivation and the results compared with those of 10 control subjects. All patients ceased to have choreiform movements for approximately 60 minutes during the insulin tolerance test. Four of the patients failed to show clinical features of stress in response to hypoglycaemia. The fasting blood glucose level and blood glucose response to insulin were similar for the two groups. However, the response of plasma cortisol (p less than 0.05) and of growth hormone (p less than 0.05) to hypoglycaemia was earlier in patients than controls, though peak responses were the same for each group. The thyrotrophin releasing hormone test revealed no difference in basal levels of thyroid stimulating hormone in either group, or in peak response to thyrotrophin releasing hormone or in the increment at 20 minutes. One of the patients had a delayed response typical of a hypothalamic disorder, whereas none of the controls had such a response. Mean free thyroxine index levels for each group were similar. There was no difference in basal prolactin level, or in the increment or in the peak level in response to thyrotrophin releasing hormone between each group as a whole or when the males and females were analysed separately. Because of small subgroups, the data from the gonadotrophin releasing hormone test were difficult to analyse, but no clear differences or obvious abnormalities emerged. Water deprivation revealed no evidence of inability to concentrate urine in either group and hence no indication of impaired antidiuretic hormone function. The study supports previous findings of altered hypothalamic function in patients with Huntington's chorea but further suggests that serotoninergic rather than dopaminergic mechanisms may be altered.