Pyridoxal phosphate Is the coenzyme of various decarboxylases involved in the formation of monoamlne neurotransmitters such as γ-aminobutyric acid, serotonin, dopamine, and norepinephrine. Adult male Sprague-Dawiey rats placed on a pyridoxine-deficient diet for 8 weeks showed significant hypertension compared with pyridoxJne-supplemented controls. Hypothalamic contents of pyridoxal phosphate, γ-aminobutyric acid, and serotonin in the pyridoxine-deficient rats were significantly lower than those in pyridoxine-supplemented controls. Hypertension was associated with sympathetic stimulation. Treatment of pyridoxine-deficient rats with a single dose of pyridoxine (10 mg/kg body weight) reversed the blood pressure to normal levels within 24 hours, with concomitant restorations of hypothalamic serotonin and γ-aminobutyric acid as well as the return of plasma norepinephrine and epinephrine to normal levels. Also, pyridoxine treatment reversed the hypothalamic hypothyroidism observed in pyridoxine-deficient rats. These results indicate an association between pyridoxine deficiency and sympathetic stimulation leading to hypertension.