The kinetic mechanism of Halobacterium halobium NAD+-glutamate dehydrogenase (EC 1.4.1.3) has been investigated at pH 9.0, 3 M NaCl and 40 degrees C in both directions, by initial rate and inhibition studies. The results of the initial rate studies indicate that the mechanism is sequential with respect to substrate addition. The inhibition patterns obtained with halophilic NAD+-glutamate dehydrogenase are not consistent with a simple ordered mechanism without modification. They can, however, be reconciled with this type of mechanism by postulating an appropriate abortive complex.