Abstract 1. 1. The ammonia production of goldfish immersed in thyroxine solution increased markedly at 21°C, though there was no effect at 14°C. The total nitrogen excreted was about 75 per cent ammonia. Thyroxine had no effect on the nitrogen excretion of trout at 14°C. 2. 2. Immersion of goldfish in thyroxine for 9–14 days produced an increase in free amino acid in liver and in muscle, no change in muscle keto acids, and a decrease in muscle creatine. 3. 3. The ammonia production of isolated goldfish gills incubated in tap water was increased 13·9 per cent whereas that of gills incubated in buffer and in saline was not significantly altered. Pretreatment with thyroxine in vivo gave a similar effect, but an amino acids subsrate was necessary for it to occur. The ammonia production of trout gills was not significantly altered by thyroxine. 4. 4. There was no difference in the respiration rates of thyroxine-treated and untreated isolated gills. 5. 5. Goldfish muscle homogenate treated with thyroxine in vitro showed an 18 per cent greater uptake of 1- 14C- DL-leucine than the untreated. Liver homogenate showed no difference.