The brain slice preparation offers a unique opportunity to study synaptic function in vitro. Employing electrophysiological methods to measure synaptic activity, we manipulated the extracellular environment of the rat hippocampal slice preparation: (1) by exposing it to different degrees of hypoxia, (2) by changing the levels of glucose, (3) lactate, and (4) H+, separately and in combination with each other. The lower the oxygen level during hypoxia and the longer its duration were, the lower was the recovery rate of synaptic function in the slice upon restoration of oxygenation. Reduction or complete depletion of glucose from the perfusion medium had similar effects, although synaptic function could recover after longer periods of glucose lack as compared with oxygen lack. Reduction in the levels of both oxygen and glucose had an additive effect on the recovery rate of synaptic function when compared with the effect of each of them alone. 'Hyperglycemic' concentration of glucose prolonged the hypoxic period slices could tolerate. Acidosis, induced either by lactic acid or HCl, had no adverse effect on hypoxic slices when the pH was held at or above 6.0 or when lactic acid concentration was below 20 mM. At 10 mM, lactic acid appeared to have a beneficial effect on hypoxic slices. Consequently, it was found that lactate can replace glucose as the sole aerobic energy substrate to support synaptic function in cerebral tissue in vitro.