We examined the effects of stimulus size and location on the mouse optokinetic response (OKR). To this end, we recorded initial OKRs elicited by a brief presentation of horizontally moving grating patterns of different vertical widths and locations in the visual field. Large-field stimuli generated large sustained OKRs, whereas visual stimuli of narrower vertical widths elicited weaker sustained responses at the later period (400-500 ms after the onset of stimulus motion). However, even stimuli of only 5 degrees vertical width elicited detectable transient responses at the initial open-loop period (100-200 ms after the onset of stimulus motion). Presenting 5 degrees -width stimuli at different vertical locations (-10 degrees to +35 degrees relative to the horizon) revealed the spatial distribution of optokinetic sensitivity across the retina. The most sensitive part of the visual field was located at +25 degrees . In addition, we examined the vertical orientation of the eye under our stereotaxic set-up. We observed the optic disc using a hand-held fundus camera and determined the ocular orientation. All eye orientations were distributed in the range of +20-30 degrees relative to the horizon (25.2+/-2.5 degrees ). Thus, the direction of the most sensitive visual field matched the angle of eye orientation. These findings indicate that the spatial distribution of visual field sensitivity to optokinetic stimuli coincides with the distribution of retinal ganglion cell density.