The competitive protein binding assay used for the measurement of thyroxine in blood serum was modified for the measurement of thyroxine in urine. Samples of urine of 1 to 5 ml volume containing 0 to 5 ng/ml could be assayed with 100% recovery, and above this range, up to 10 ng, recoveries were higher due to non parallelism with the standard curves. Tests carried out using porcine serum albumin indicate that results obtained by the method are not likely to be affected by proteinuria. The cross reaction with triiodothyronine was 25%. Analysis of urine samples stored at 25°C gave higher values than those stored at +4°C or — 20°C over similar periods. These increases at 25°C were of the same magnitude as those obtained by acid hydrolysis of urinary thyroxine conjugates.