Rats received inhalation exposure to trichloroethylene (TriCE) or tetrachloro-ethylene (TetraCE) at concentrations of 200, 400 and 800 ppm for a month. Effects of the exposure on free amino acid content of midbrain were investigated by high-performance liquid chromatography. (1) A component on the chromatogram which contains glutamine, threonine and serine was greatly increased by the exposure to TetraCE in a concentration-dependent manner. By TriCE, it was significantly in-creased at 800 ppm. (2) Glutamate showed a tendency to decrease after the exposure to TetraCE. TriCE at 800 ppm significantly decreased glutamate content. (3) TriCE at 400 ppm significantly increased glycine content. (4) TetraCE decreased GABA, but on the contrary TriCE increased it. Both changes were not large and statis-tically not significant. (5) Both TetrCE and TriCE caused no significant change in taurine, aspartate and alanine contents. Exposure of rats to TetraCE and TriCE produced increase in some inhibitory putative neurotransmitter substances and decrease in some excitatory ones.