There are reports that insulin may protect neurons from the effects of ischemia. The mechanisms for this protection are not fully understood. We studied the extracellular levels of glutamate and GABA in insulin-treated animals exposed to transient forebrain ischemia under normoglycemic and hypoglycemic conditions. In vivo microdialysis technique was used to collect extracellular fluid from the CA1 region of the hippocampus. There was a significant increase in GABA levels in the two insulin-treated sub-groups compared with the controls. GABA levels were < 1 pmol/10 microliters in three 10 min collections prior to ischemia in all the groups. It increased from 11.1 +/- 3.5 pmol/10 microliters in the conrol group to 47 +/- 5 (P < 0.001) in the insulin-treated hypoglycemic group and up to 47.2 +/- 9.3+ (P < 0.005) in the insulin-treated normoglycemic group (two-way ANOVA with repeated measures). Ischemia resulted in an increase in the glutamate levels. The glutamate levels returned to baseline within 30 min of the insult. There were no significant differences in the glutamate levels in three groups. The increase in GABA concentrations in the extracellular space may result in the inhibition of CA1 pyramidal neurons. This may be a possible mechanism of neuronal protection in animals treated with insulin (with or without being hypoglycemic) during ischemia.