Serum thyroxine (T4), triiodothyronine (T3), the T3 resin uptake test (T3RU) and TSH were measured in 85 clinically euthyroid patients on regular haemodialysis; sera were collected immediately before dialysis. 32% of patients had goitres. Serum T3 was below the normal range in 34% and T4 in 39%; the T3RU was in the high normal range in most. Serum TSH was slightly elevated in 13 patients. In 20 patients dialysed for 3 months or less the serum T4 was normal in 18 and the free thyroxine index (FTI) was normal in every case; mean T4 and FTI values were significantly lower in patients dialysed for more than 3 months. 16 patients were studied prospectively on two occasions 14 months apart; those initially dialysed for less than 1 year showed a significant fall in mean T4 and FTI values after a further 14 months of dialysis, while levels were unchanged in those dialysed initially for more than a year. Thyroid hormone levels were measured before and immediately after a single dialysis in 57 patients. Dialysis caused significant acute elevations of T3, T4, and FTI, but had no consistent effect on TSH. The TSH and prolactin responses to TRH were investigated in 18 male patients: the mean maximal TSH increment was significantly lower than in controls and peak responses were subnormal in 5 patients. Basal serum prolactin levels were elevated but peak responses were normal. It is concluded that, in patients on regular haemodialysis: (1) goitre may be frequent; (2) in vitro thyroid function tests were often subnormal just before a period of dialysis, but were usually normal immediately after the dialysis; (3) serum T4 levels fall rapidly in the first year of dialysis; (4) the pituitary responsiveness to low serum thyroid hormone levels and to exogenous TRH is often abnormal in dialysed patients, although they appear to be euthyroid.