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Thyroid hormones exert a crucial role on trophic events of the central nervous system during development, adulthood, and ageing. The deficiency of thyroid hormones could also produce a deficiency in neurotransmission in the hippocampal region. Kainic acid (KA) has become an important tool for studying functions related to excitatory amino acid transmission in mammals. Its neurotoxic effects on the pyramidal neurons of the CA3 hippocampal region are well known. We have examined the neurotoxicity of KA on these cells in hypothyroid rats. The hypothyroid state was induced by administration of methimazole. After 4 weeks of treatment, KA was administered once intraperitoneally at doses of 0, 1, 2.5, and 5mg/kg to the hypothyroid group, and 0 and 5mg/kg to the euthyroid group. In the euthyroid group, KA reduced the neuronal density in the CA3 hippocampal region, and in the hypothyroid rats with no administration of KA, the neuronal density of the CA3 hippocampal region is reduced also. Administering KA in hypothyroid rats did not reduce the number of CA3 pyramidal cells.