L-Glutamate decarboxylase (GAD) activity of Clostridium perfringens (ATCC 8009) cells grown in various culture conditions was investigated. Remarkable variations of GAD level occur during the growth cycle in thioglycollate broth. These changes are affected by the pH of the culture medium. Addition of alkali to the culture media results in decrease of cell GAD activity, whereas increase of enzyme level occurs only in cells growing in unbuffered media. The results indicate that the mechanism regulating the GAD levels is sensitive to the changes of pH (or buffering substances) rather than to the steady pH values. Neither repression by glucose nor induction by L-glutamate was observed. Moreover, high concentrations of the free amino acid substrate in the culture media considerably decrease cell GAD activity, owing to the buffering effect of the amino acid. The molecular mechanism supporting the variations of GAD activity during the growth cycle of the cells were investigated and tentatively related to the structural and functional properties of the pure enzyme. It is shown that the drop of GAD activity during the lag phase is due to protein breakdown. Evidence is presented suggesting a control of protein degradation by its quaternary structure. Data are also reported supporting de novo synthesis of GAD during the late logarithmic phase of cell growth. Finally, the possible role of GAD as part of the pH regulation system of C. perfringens cells is discussed in relation both to physiologic conditions of the bacterial cell and to the molecular mechanisms regulating the GAD activity in vivo.