The increased activity of carbamyl phosphate synthetase I [carbamoyl-phosphate synthase (ammonia); ATP: carbamate phosphotransferase (diphosphorylating), EC 126.96.36.199] in tadpole liver observed during thyroxine-induced metamorphosis was markedly inhibited by intraperitoneal injection of the microbial protease inhibitor antipain (0.1 micrometermol/g of body weight, twice daily). A somewhat less than maximal inhibition was seen when antipain was given only during the first 2 days of thyroxine treatment. On the other hand, little inhibition was observed when the inhibitor was given after the third or fourth day of thyroxine treatment. Antipain also inhibited thyroxine-induced increases of ornithine transcarbamylase (EC 188.8.131.52), arginase (EC 184.108.40.206), and succinate-cytochrome c reductase (EC 220.127.116.11) activities. Among other microbial protease inhibitors tested, chymostatin was nearly as effective as antipain, leupeptin was less effective, and pepstatin was ineffective. Analysis of the total liver protein and of the immunoprecipitate by sodium dodecyl sulfate/polyacrylamide gel electrophoresis showed that the inhibition was due to decreased amount of the enzyme protein. Antipain had no significant effect on leucine incorporation into total protein of tadpole liver. These results indicate the involvement of a proteolytic step in the pretranscriptional events in thyroxine-stimulated enzyme induction.