Metabolic clearance rates and production rates for thyroxine (T4), triiodothyronine (T3) and reverse T3 (rT3) were determined by pulse injection kinetic techniques in newborn sheep between 7 and 14 days of age. The mean metabolic clearance rate for T3 (39.6 +/- 4.5 l/m2 per day) was significantly greater than that for T4 (7.25 l/m2 per day, P < 0.001). The mean value for rT3 (27.5 +/- 2.8 l/m2 per day) also exceeded that for T4 (P < 0.001) but did not differ from that for T3. The mean production rate for T3 (95.6 +/- 5.8 micrograms/m2 per day) was significantly less than that for T4 (511.4 +/- 71.7 micrograms/m2 per day, P < 0.01). The mean value for rT3 (28.6 +/- 1.7 micrograms/m2 per day) was significantly less than respective values for T4 (P < 0.01) and T3 (P < 0.001). Calculated thyroid gland secretion accounted for 42.3 micrograms/m2 per day of T3 production and 4.7 micrograms/m2 per day of rT3 production. Calculated peripheral conversion from T4 accounted for 53.3 micrograms/m2 per day (56%) of the production of T3 and 23.9 micrograms/m2 per day (84%) of that of rT3. The metabolic clearance rate for T4 and the production rate for T4 and T3 in the newborn were greater than those measured earlier (Chopra, Sack & Fisher, 1975) in the fetus or adult ewe. The metabolic clearance rate of T3 in the newborn was less that the fetal value but similar to that measured in adult animals. The newborn value for rT3 was greater than that in the fetus and similar to that in the adult ewe. The production rate of rT3 was lower than that previously measured in the fetus but similar to that in adult ewes. Our results suggest that the transient chemical thyrotoxic state characteristic of the newborn is accountable on the basis of increased thyroid gland secretion of T4 and T3 as well as increased T3 production from T4 via peripheral monodeiodination.