Intravenous bolus administration of oxygen 15-labeled water and positron emission tomography were used to measure changes in brain blood flow induced by two modes of photic stimuli over a wide range of repetition rates. These stimuli (patterned-flash and reversing checkerboard) were chosen in order to determine whether stimulus luminance or stimulus frequency was responsible for previously observed increases in blood flow in the striate cortex during photic stimulation. The response curves of blood flow change as a function of stimulus rate were nearly identical for both stimuli. These results suggest that elementary stimulus variables, such as repetition rate, can have a major effect on local cerebral responses, as measured with positron emission tomography and other radiotracer methods.