1. The present investigation explored, in thirty-four anaesthetized cats, the blood flow changes at the optic nerve head elicited by sinusoidally modulated photic stimuli. 2. The stimuli were achromatic, diffuse and had 30 deg diameter field size; the stimulus frequency was varied from 0 to 100 Hz, modulation depth from 0 to 100% and mean retinal illuminance up to 50,000 trolands (td); the blood flow was measured with a near-infrared (810 nm) laser Doppler flowmeter. 3. At various frequencies, modulation depths and mean retinal illuminance, sinusoidal flicker stimulation always caused an increase in blood flow at the optic nerve head relative to steady stimulation. 4. The frequency response and temporal contrast sensitivity function of the blood flow changes had a bandpass shape; the high-frequency slope of the frequency response was 3 decades (dec) per decade and that of the temporal contrast sensitivity function was 1.7 dec per dec, close to the slope for cat 'on' ganglion cells (2.6 dec per dec). 5. In most cats, the magnitude of the increase in blood flow was a sigmoidal function of modulation depth; in the remainder, the relationship was close to linear. 6. The threshold of blood flow changes varied with respect to mean retinal illuminance similar to Ferry-Porter's law and the photopic linear slope was 50 Hz dec-1. 7. In comparison with reported psychophysical and electrophysiological responses elicited by similar stimulations, the results of the present study resemble more those obtained from ganglion cells than those from electroretinograms, visual-evoked potentials and psychophysics. It is suggested that the blood flow changes at the optic nerve head are induced by the activity of ganglion cells.