The aim of this study was to determine if immunoreactivity for phosphate activated glutaminase (PAG), an enzyme involved in the biosynthesis of glutamate and a putative marker for neurons that use glutamate as a neurotransmitter, is present within respiratory neurons in the ventrolateral medulla oblongata. Intracellular recordings were obtained from neurons in the ventrolateral medulla of adult anaesthetised Sprague-Dawley rats. Neurons with a respiratory-related modulation of their membrane potential were filled with Neurobiotin (Vector, CA). After histochemical processing, sections of brainstem were examined by fluorescence and light microscopy. Some FAG immunoreactivity was found in all of the four types of respiratory neurons examined. FAG immunoreactivity was graded as strong or weak. (1) Of six inspiratory neurons in the rostral ventral respiratory group five were strongly FAG immunoreactive and one was weakly FAG immunoreactive. (2) Of six expiratory neurons in the caudal ventral respiratory group five were strongly FAG immunoreactive while one was weak. (3) Seven motoneurons in the nucleus ambiguus were all strongly FAG immunoreactive. (4) Five neurons in the Botzinger area were examined. Four were weakly FAG immunoreactive while one contained strong FAG immunoreactivity. These data demonstrate a heterogeneity of FAG immunoreactivity amongst brainstem respiratory neurons.