1. To understand the mechanisms which lead to acute neuronal swelling during anoxia, we studied the ionic movements of Cl- and Na+ during O2 deprivation in the hypoglossal (XII) neurons of rat brain slices using double-barrelled ion-selective microelectrodes. 2. Baseline extracellular Cl- and Na+ activities ([Cl-]o, [Na+]o) were 128.3 +/- 7.4 and 150.0 +/- 3.4 mM respectively (n = 12) in the adult. Similar baseline values were obtained from neonatal brain slices. 3. During a period of anoxia (4 min), [Na+]o decreased by about 40 mM in adult slices while [Na+]o did not show any significant change in the neonate (n = 12). Although anoxia induced a significant decrease of [Cl-]o in both adult and neonate, [Cl-]o dropped 7 times more in the adult than in the neonate (n = 12). 4. Intracellular Cl- activity ([Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i) was studied in twenty-seven adult hypoglossal cells. All showed an increase in [Cl-]i with O2 deprivation. Detailed analysis carried out on ten hypoglossal neurons showed a baseline [Cl-]i of 11.4 +/- 4.5 mM and an increase in [Cl-]i by 20.6 +/- 7.2 mM during O2 limitation. 5. Baseline [Cl-]i in neonatal XII neurons was similar to that of the adult. Anoxia, however, produced an increase in [Cl-]i by only 4.5 +/- 2.4 mM (n = 7). This increase in [Cl-]i was significantly less than that in the adult (P less than 0.001). Prolonged anoxia (6-12 min) in the neonate led to a more substantial increase in [Cl-]i, an observation consistent with the decrease in [Cl-]o after prolonged O2 deprivation. 7. We conclude that during anoxia: (1) intracellular [Cl-] increases in the adult and this most likely occurs because of entry of extracellular Cl- into the cytosol and (2) there is a major maturational difference in mechanisms regulating Cl- and Na+ homeostasis between newborn and adult brain tissue. We speculate that these mechanisms may account, at least partially, for the relative tolerance to anoxia in the newly born.