Glutamate dehydrogenase (GDH) was found to be inducible in 28-day-old Sprague Dawley rats by single injections of cortisol hemisuccinate (10 micrograms/g BW), glucagon (3 micrograms/g), (Bu)2cAMP (100 micrograms/g), T4 (20 micrograms/g), or T3 (10 micrograms/g). Similar responses were not seen in either younger or older rats. The maximum response to cortisol was seen in 2 h, and to glucagon and (Bu)2cAMP in about 8 h. Both effects were inhibited by actinomycin D and cycloheximide, and were accompanied by increased incorporation of 3H-leucine into immunoprecipitable GDH, indicating the probable involvement of de novo enzyme synthesis. The thyroid hormones gave bimodal responses, with one peak at 2 h and another at about 12 h. The first peak was not inhibited by either actinomycin D, cycloheximide, or puromycin, and involved no increased incorporation of 3H-leucine into immunoprecipitable enzyme. This suggests that the effect does not involve de novo synthesis of the enzyme. The loss of label from GDH indicates a half-life for the enzyme of about 9-12 h, which is not affected by cortisol or (Bu)2cAMP.