1. Hand heat elimination was measured during central body heating or cooling to evaluate the sensitivity of thermoregulation in patients exhibiting Raynaud's Phenomenon. 2. Patients with Raynaud's Phenomenon responded to central heating and cooling very much as did normal subjects. 3. Patients with Raynaud's Phenomenon and associated connective tissue disease do not vasodilate in the hand in response to central heating as much as normals. These findings are compatible with the concept of a local structural defect in the blood vessels of the hand. 4. The nature of the abnormality causing Raynaud's Phenomenon is not apparent but it does not appear to be due to an increased sensitivity of the central thermoregulatory system.