Changes in neurotransmitter or neuromodular activity and levels have been thought to be responsible for some of the behavioral characteristics observed in patients with thyroid disease. To investigate these relationships, taurine, a proposed inhibitory neurotransmitter or modulator, was measured in platelets, which may serve as biochemical model for synaptosomes. Blood of euthyroid, hypothyroid and hyperthyroid patients was collected. Thyroxine was determined by RIA. Platelets were separated using differential centrifugation techniques. The taurine in platelets was separated by ion exchange chromatography and was assayed spectrophotometrically as a product produced by coupling with dinitrofluorobenzene. In hypothyroid, untreated hyperthyroid and euthyroid individuals, taurine/platelet and number of platelets/ml of blood were inversely related. In propranolol-treated hyperthyroid patients, such a relation was not found. Following initial treatment with either antithyroid drugs (i.e. methimazole or propylthiouracil) and/or propranolol, taurine/platelet returned to euthyroid levels while plasma thyroxine levels remained elevated. These data implicate a relationship between taurine in platelets and the thyroid state in man.