The serum TSH was measured by radioimmunoassay after i. v. injection of 500 .Eg of TRH in 9 undialysed patients and 12 dialysed patients with chronic renal failure and in 6 healthy subjects. The basal level of the serum TSH in dialysed patients with chronic renal failure was significantly higher than in healthy subjects. The serum TSH rose to maximum level 30 minutes in healthy subjects and 60 minutes in patients with chronic renal failure after TRH injection. The mean maximum increment of the serum TSH in healthy subjects was significantly higher than that in patients with chronic renal failure. Although the serum TSH in healthy subjects returned to basal level 120 minutes after TRH injection, the recovery to basal level was very much retarded in patients with chronic renal failure. These findings may indicate a reduced response of the pituitary to TRH and decreased turnover of TSH in patients with chronic renal failure.