Serum free thyroxine (FT4) concentrations were shown to be significantly reduced at 36-38 weeks normal pregnancy, both as measured FT4 by the Amerlex method (P less than 0.001), and as calculated FT4 (P less than 0.001) using accepted molecular weight and affinity constant data for the binding proteins. Serum FT4 concentrations as determined by the Immophase method were normal at 36-38 weeks normal pregnancy. All methods gave normal serum FT4 concentrations in subjects taking the oral contraceptive pill. FT4I and FT3I, derived using the MAA T3 uptake-value, were higher than normal at 36-38 weeks pregnancy (P less than 0.001), whereas T4/TBG and T3/TBG were both reduced (P less than 0.001). The observation that serum FT4 concentrations may fall in late pregnancy, as demonstrated both by the Amerlex radioimmunoassay technique and by calculation, suggests that circulating FT4 may not be the sole determinant of thyroid status at this time. From a practical viewpoint, it is important to note that currently available direct and indirect methods of assessing serum FT4 concentrations produce different patterns of change in pregnancy.