The purpose of this work was to estimate the 24 h urinary excretion of free and conjugated triiodotironine (T3) using a direct radioimmunoassay and enzyme hydrolysis. Mean urinary values of free and total T3 (mean +/- 1 SD) in euthyroid controls were 2074 +/- 673 and 2819 +/- 809 pmol/24 h respectively. In patients with hyperthyroidism, values of free hormone were about 4.2 times higher than the mean value of the euthyroid controls, and about one-third in patients with hypothyroidism. These results show this measurement to be useful as an indicator of thyroid function. Mean renal clearance of free T3 was 211.6 +/- 62.8 ml/min (mean +/- 1 SD) in euthyroid controls, 260.8 +/- 87.5 ml/ml in hyperthyroid patients and 229 +/- 98.7 ml/min in hypothyroid patients. The data show that T3 renal clearance is, in all cases, greater than glomerular filtration rate, suggesting tubular secretion of T3.