Thermal inactivation of homogeneous NAD-kinase and its partially purified preparation containing NAD-kinase in complex with glutamate dehydrogenase has been studied. The complex is more resistant to thermal inactivation in comparison with isolated NAD-kinase; its inactivation consists in irreversible dissociation preceded by a period of constant activity. No period of constant activity is observed when homogenous NAD-kinase is subjected to thermal inactivation, which represents a two-step process. At the first stage tetramers reversibly dissociate into dimer, while at the second stage the dimers are denatured. The values of activation energy of dissociation and denaturation are equal to 40 and 52 kcal/mol, respectively.