L-Thyroxin was administered orally to 7 subjects (2 normal, 2 congenitally hypothyroid and 3 thyroidectomized subjects; all subjects were euthyroid at the time of the study and 1 was pregnant) in a single weekly dose; serum thyroxin (T4) and triiodothyronine (T3) concentrations were measured throughout the week. A dose of 1.4 mg/m2 leads to elevated T4 concentrations during the greater part of the week in all subjects, whereas at a dose of 1.1 mg/m2 T4 levels were elevated for only 2 days and remained in the high normal range for the rest of the week. T3 concentrations rose nearly as rapidly as T4 concentrations and tended to remain in the high euthyroid range throughout the week despite elevated T4 concentrations. The absorption peak occurred 2–4 hr after ingestion.