1. We confirm the observation of Bürk (1965) that Neurospora crassa NADP-linked glutamate dehydrogenase normally exists in an inactive form below pH7.0 and in a fully active form above pH8.0 in either tris or orthophosphate buffer. At pH7.4 the enzyme is about half activated at 25 degrees . 2. The variety of the enzyme produced by the mutant am(2l) shows a similar behaviour except that the transition is shifted about one pH unit in the alkaline direction. 3. The am(2l) enzyme has previously been reported to be activated by brief warming to 30 degrees in phosphate buffer at pH8.0. The wild-type enzyme shows a similar effect at pH7.0. In tris buffer this effect is much less pronounced. 4. The am(2l) enzyme is extremely unstable at 47 degrees at pH7.0; its stability is somewhat greater at lower pH, and is markedly increased by increasing the pH in the range 7.0-8.7. The wild-type enzyme also shows an indication of a stability minimum at pH7.0, but a temperature of 60 degrees is needed for a measurable rate of inactivation. 5. The inactive form of the enzyme is much more subject to thermal irreversible denaturation than is the active form.