Urinary excretion of the thyroid hormones (TH) L-thyroxine (T4), and 3,5,3'-triiodo-L-thyronine (T3) and their derivatives was studied in urinary-catheterized rainbow trout (mean wt. 227 g) fasted for 2 days under a 12-hr L:12-hr D photocycle in running water (12 degrees). Catheterized trout were intracardiac-injected with [125I]T4 (*T4) or [125I]T3 (*T3) and urine was collected as 2-hr fractions over 48 hr. The 125I corresponding to I-, TH conjugates (sulfates and glucuronides), or TH was separated by LH-20 column chromatography and HPLC. Urine production (daily mean, 72.5 ml/kg/day) was lowest during the scotophase and doubled at the start of photophase, causing acute fluctuations in excretion of 125I-labeled materials, and implying dependence on glomerular filtration rate. During the first 48 hr, 8.2% of *T4 (I-, 3.6%: TH conjugates, 1.7%; TH, 2.9%) and 6.7% of injected *T3 (I-, 1.8%; TH conjugates, 2.5%; TH, 2.4%) was excreted in urine; 32.6% (*T4) and 26.4% (*T3) was in the gall bladder; 45.5% (*T4) and 45.7% (*T3) were in the remaining carcass; and 13.7% (*T4) and 23.7% (*T3) were lost via other routes. We extrapolate that about 15% of *T4-injected and 12% of *T3-injected total radioactivity would be excreted ultimately in urine, with 8.4% (*T4) and 9.0% (*T3) as TH or their conjugates. Neither a T4 nor a T3 challenge (20 ng/g) influenced the amount of radioactive loss in urine over 48 hr. We conclude that the urine is a significant route for excretion of TH and their conjugates, and that urinary TH loss depends to a large extent on the rate of urine production.