Hippocampal slices were prepared from 4-day, 7-day and 10-day-old, and adult rat brains. Energy use rate and oxygen consumption rate were determined in each slice. Neural activity (population spikes, PS) was recorded in the CA1 pyramidal cell layer and correlated with ATP and creatine-P levels during deprivation of oxygen and/or glucose. The energy use rate of respective slices was 1.09-P mmol/gDNA/min in 4-day-old, 2.93 in 7-day-old, 3.42 in 10-day-old and 4.75 in adult rat, respectively. The oxygen consumption rate for slices from 4-, 7-, and 10-day-old and adult rats was 0.13, 0.24, 0.47 and 0.8-O2 mmol/gDNA/min, respectively. Thus energy use rate as well as oxygen consumption rate increased during development and the difference in the time course of the increase between energy use and oxygen consumption indicated clearly that the energy production in the immature brain is mainly supported by anaerobic glycolysis whereas in mature rat by aerobic metabolism. The correlation between neural function (amplitude of PS) and the level of ATP of each slice during oxygen and/or glucose indicated that the presence of glucose is essential for the preservation of the neural activity in addition to the maintenance of ATP levels in the slice.