Thyroid function was studied in healthy premature and term infants between 12 hours and 3 months of age. T4 and FT4I followed parallel courses in both groups; during the first 45 days, however, the values were significantly lower in premature infants under 34 weeks' EGA than in term infants (P less than 0.001). The post-delivery peak in TSH concentration (mean +/- SD) was 71.8 +/- 19.2 microunits/ml in the premature infants. In five premature infants, injection of TRH elicited a TSH increment of 29.4 +/- 20.7 microunits/ml at 30 minutes. T3 concentration was not significantly different in premature and term infants.