Thyroid function tests were performed in nine clinically euthyroid, chronic-renal-failure (CRF) patients on continuous ambulatory peritoneal dialysis (CAPD), and the results were compared with similar tests performed on normal controls and eight patients on maintenance hemodialysis (HD). As reported earlier in untreated patients with CRF and those maintained on HD, our patients on CAPD had markedly reduced total tri-iodothyronine (T3) concentration. Levels of serum thyroxine (T4), and serum free T4 estimated by the microencapsulated antibody technique were reduced in both groups of patients but were in the hypothyroid range only in the HD group. However, in keeping with the clinically euthyroid status of these patients, thyroid stimulating hormone levels were within normal limits. Finally, both groups of patients had low normal reverse T3 levels. These data confirm the presence of abnormalities in in vivo thyroid function tests in patients with CRF maintained by different modes of dialysis. The significance and mechanism of these abnormalities remains speculative.