Free thyroxine (FT4) and free triiodothyronine (FT3) were measured by column adsorption chromatography followed by radioimmunoassay in dried blood spots on filter paper in euthyroid subjects, hyperthyroid and hypothyroid patients, and in subjects with TBG excess. The sensitivity (B/T% = 95%) was 1.5 pg/ml (working range 1.5-46.4 pg/ml) for FT4 and 1.5 pg/ml (working range 1.5-32.0 pg/ml) for FT3. Intraassay coefficient of variations (CVs) ranged 4.4-8.8% for FT4, 8.7-10.1% for FT3; interassay CVs varied from 8.9-9.0% for FT4, 9.3-10.4% for FT3. FT4 and FT3 values found in dried blood spots were highly correlated with the corresponding values in serum (r = 0.97, P less than 0.001 for FT4; r = 0.96, P less than 0.001 for FT3). FT4 concentrations in dried blood spots ranged 8.1-20 pg/ml in euthyroid subjects, 19.4-60.0 pg/ml in hyperthyroid patients, less than 1.5-7.1 pg/ml in hypothyroid patients, 7.8-18.8 pg/ml in euthyroid subjects with TBG excess. FT3 values in dried blood spots ranged 2.5-5.8 pg/ml in euthyroid subjects, 7.1-30.0 pg/ml in hyperthyroid patients, less than 1.5-2.8 pg/ml in hypothyroid patients, 2.5-5.2 in euthyroid subjects with TBG excess. The results of the present study, while confirming previous data on FT4 determination in dried blood spots, represent the first report on FT3 measurement in the same system, thus allowing a more complete assessment of thyroid status made by mail at the expense of few drops of blood.