A small spot of flashing light could trigger interictal waves when the position of the spot in the visual field corresponded to the position of penicillin application on the primary visual cortex. The circumscribed visual field area was called the activation field. The size of the activation field was primarily determined by the magnification factor at the site of penicillin application and the size of the applicator. The probability of epileptiform triggering by a spot within the activation field was determined as a function of time following a spontaneously occurring interictal wave. The latency between the stimulus transients and interictal waves was consistently a function of the response probability, not the stimulus parameters. Apparently, triggering is primarily determined by the neural volley which is projected totally or in part to epileptogenic cortex by the geniculocortical axons. The activation field represents the set of visual field sites which excite the cortical afferents to epileptogenic cortex.