Pyrimidine-repressible carbamyl phosphate synthetase P was synthesized in parallel with aspartate transcarbamylase during growth of Bacillus subtilis on glucose-nutrient broth. Both enzymes were inactivated at the end of exponential growth, but at different rates and by different mechanisms. Unlike the inactivation of aspartate transcarbamylase, the inactivation of carbamyl phosphate synthetase P was not interrupted by deprivation for oxygen or in a tricarboxylic acid cycle mutant. The arginine-repressible isozyme carbamyl phosphate synthetase A was synthesized in parallel with ornithine transcarbamylase during the stationary phase under these growth conditions. Again, both enzymes were subsequently inactivated, but at different rates and by apparently different mechanisms. The inactivation of carbamyl phosphate synthetase A was not affected in a protease-deficient mutatn the inactivation of ornithine transcarbamylase was greatly slowed.