Temporal summation as the relation of threshold intensity to presentation time of a stimulus has been tested in different retinal locations under photopic and mesopic conditions in 35 normal subjects of all ages. The characteristics of temporal summation is quite independent of age, retinal location and adaptation of the eye. The only variable is the time of transition from complete to absent temporal summation, ranging from 32 msec in photopic foveal vision to more than 320 msec in mesopic peripheral vision