The increase in the content of taurine in the central nervous system (CNS) of amyotrophic lateral sclerosis (ALS) may probably be related to the impairment of folate cycle. In addition, the appearance of pyramidal sign in patients with hyperthyroidism as well as impairment of folate metabolism in thyropathies have been reported. This study was undertaken to investigate whether excess or deficiency of thyroid hormones could induce changes in free amino acids and folate cycle in CNS. Wistar male rats weighing approximately 370 g were used. Forty rats were thyroidectomized, and twenty were sham-operated as control under pentobarbital anesthesia. After 22 to 24 weeks, these animals were sacrificed by decapitation and the blood was collected in a heparinized and a non-heparinized container. Brain, spinal cord, liver, skeletal muscle, erythrocytes, plasma, and serum were stored at -80 degrees C until analyzed. To make hyperthyroidism in rats, triiodothyronine (T3) solved in alkaline 0.9% saline (10 micrograms of T3/100 g of body weight) were injected intraperitoneally to 16 rats six times a week, totally thirty times. Only alkaline 0.9% saline were injected to sixteen controls. These rats were sacrificed by the same way described above and each material was also stored at -80 degrees C. The content of taurine was changed in various tissues including CNS in thyroidectomized and T3-injected rats. The significant increase of taurine was seen in spinal cord and skeletal muscle of thyroidectomized rats. On the other hand, the significant decrease of taurine was observed in brain, liver, and skeletal muscle of T3-injected rats. The folate level was increased only in liver of thyroidectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)