a control systems approach has been made to the investigation of the peripheral vasomotor control system. the thermal vasomotor control of digital bloodflow has been investigated in 46 subjects fourteen were controls, 26 were suffering from raynaud's phenomenon and the rest had suspected malfunctions of the sympathetic nervous system. to assess the frequency response of the system a square-wave temperature stimulus was applied externally to one hand while the blood flow in the forefinger of the contralateral hand was recorded the frequency and phase response of the control system was investigated between 0. 1 and 0. 0125 hz. the results show that patients suffering from raynaud's phenomenon have a different frequency response from the normal control group. a mathematical model of the control system was devised, and its use has indicated that the differences between patients with raynaud's and normals may be explained in terms of the gain and time delay in the control loop using this model and the data obtained it should be possible to develop a new method of differential diagnosis. 13 refs