We investigated the correlation between N-acetylaspartate (NAA) level and neuronal density in the hippocampal CA1 region of the brain after occlusion of both common carotid arteries for 5 minutes and reperfusion for 3 hours to 4 weeks in gerbils with and without ischemic preconditioning (tolerance). Animals were divided into four groups--the sham operated group, the nonpreconditioning (non-p) group, the single-preconditioning (single-p) group with 2-minute ischemia once 2 days before 5-minute ischemia, and the double-preconditioning (double-p) group with 2-minute ischemia twice 2 days before 5-minute ischemia (n = 6 for each group). The CA1 region was dissected out from freeze-dried sections for high-performance liquid chromatographic assay of NAA, and adjacent sections were stained with cresyl violet for measurement of the neuronal density. Both NAA (pmol/microg dry weight) and the neuronal density (cells/mm) decreased in the non-p group after 3 days (NAA = 24.0 +/- 3.0; neuronal density = 65 +/- 38 cells/mm) and 7 days (NAA = 17.9 +/- 2.5; neuronal density = 20 +/- 15 cells/mm) and in the single-p group after 7 days (26.4 +/- 3.0, 106 +/- 30) compared with the control group (NAA = 32.9 +/- 3.0; neuronal density = 203 +/- 9 cells/mm). There was no decrease in the double-p group. The NAA level and the neuronal density showed a good linear correlation. The regional NAA level may be used as an index of neuronal viability.