Stimulation in cats of the hypothalamus in combination with reflex stimulation of the sciatic nerve or cutaneous nerve endings was used for the study of spatial summation of the sympathetic system with the nictitating membrane, the blood pressure and the heart rate as indicators. If subthreshold stimuli are chose they become effective in combination; if suprathreshold stimuli are used they produce effects which are greater in combination than the sum of the effects of the stimuli applied singly. When administered repeatedly at suitable intervals (temporal summation) they become increasingly effective. These summation processes may occur in all of the above mentioned indicators or in one only. The latter case proves the occurrence of aPartial activation of the sympathetic system.