1. The effects of hypothyroidism in utero , and continuing into postnatal life, on the central turnover of catecholamines, noradrenaline and dopamine were studied in rats. 2. Rats were rendered hypothyroid in utero by treating pregnant females with methimazole in the drinking water. In two groups goitrpgen treatment continued for 3 or 10 weeks postnatally. 3. Methimazole treatment in utero did not produce significant changes in noradrenaline, dopamine or tyrosine content in either the hypothalamus or striatum. 4. Three weeks' postnatal treatment reduced tyrosine specific radioactivity in the anterior hypothalamus and dopamine specific radioactivity in the striatum. Ten weeks' treatment increased dopamine content and reduced noradrenaline synthesis in the mediobasal hypothalamus and a reduction in tyrosine content in the anterior hypothalamus. 5. These data suggest that hypothyroidism restricted to intrauterine life does not produce permanent changes in adult catecholamine neuronal function. Long term hypothyroidism produced localized changes, suggesting a specific rather than general effect.