The optokinetic reflex undergoes qualitative changes during the first postnatal weeks in kittens or months in human babies. Under monocular stimulus conditions, a clear preference for temporonasal stimulus directions at moderate velocities is replaced by a symmetrical, broad velocity range horizontal optokinetic nystagmus (OKN) over these periods. Evidence is presented for the cat that development changes in OKN can be related to maturation of neuronal response properties in the nucleus of the optic tract (NOT) in the pretectum. NOT cells in 3-week-old kittens are already direction-selective but all of them are exclusively or predominantly driven by the contralateral eye. Only starting with the 4th week of life NOT cells become more binocular and respond to a broader spectrum of stimulus velocities. This step in maturation coincides with the time when the cortical input to the NOT becomes functional.